I've always pretty much assumed that magic items do count for feat prereqs--of course noting that if you lose the boost, you lose access to the feat (as you would if you lost your stats permanently in some other fashion.)
On reflection, though, this should *probably* be treated the same way that Intelligence is now treated for skill points. In 3.0, there was this fuzzy "You get the skill points from a magic item that you used the whole level" sort of thing. In 3.5, they got rid of that (which is a good thing, really.) So, in 3.5, I think that the same should apply to feats.
Reasons to avoid allowing magic items to influence feats:
1) If magic items can be used to help satisfy feat prerequisites, it's *much* harder to create new characters at higher level. You have to think about not just what skills and abilities were available at each level, but also what magic items were owned by the character at each level.
2) The good old "lose the magic item, lose the benefits of the feat" problem--magic items are a lot easier to lose... "Whoops, natural one on saving throw... cloak of charisma +6 goes up in smoke."
In the interest of sanity, I think that there are a couple of ways to approach these problems, should they actually come up in game. House ruling either way works and is sane:
1) You can take any feat any time you want. You do not gain the benefit from feats that you do not have the pre-requisites for, but you can gain the feat. (i.e. if you have the Power Attack feat and a Strength of 12, you possess the training to perform a Power Attack, but you aren't strong enough to perform it successfully without aid from magic, or some bodybuilding to push Strength up to 12.) I'd say that this only applies for ability score prerequisites--skill rank and feat prerequisites must be satisfied to receive the training.
Taking this approach simplifies character building somewhat, especially for higher levels (where you might spend character-creation money on Tomes to increase ability scores) and makes the rules consistent that it's what abilities you have *now* that matter. The prerequisite on ability represents abilities you must have to successfully use the technique.
This is probably the most sane approach to take if you want to allow magic items to satisfy feat prerequisites--you no longer have to worry about when an ability was boosted by magic, but you do have to keep worrying about magic item destruction. (But you do have to work to stay reasonable when creating higher level characters: it's not reasonable for a character to take three feats at 1st-level that he'd never be able to use until 8th-level.)
2) You can only take feats when you satisfy the prerequisites without any sort of magical (or psionic, or whatever) aid. Once you have a feat, you can never lose its benefits. (i.e. if you had Strength 13, and took the Power Attack feat, you can continue to Power Attack even if some effect later decreases your Strength to below 13.) This makes ability score prerequisites identical to feat and skill rank prerequisites: there's no way to lose skill ranks or lose feats, and in this case, losing ability scores (for this single purpose) is also treated as impossible.
Taking this approach simplifies day-to-day playing when ability drain and ability damage are on the table. This is especially important if you use Psionics, since there are a good number of abilities that can serious smack every ability score around. (And, there's another Psionics-related reason to prefer this solution, for which see below.) In this case, the prerequisites represent abilities required to benefit from the training for the feat--but once you possess the training, you receive its benefits even if something acts to decrease your abilities.
This is probably the most sane approach if you are not going to allow magic items to satisfy prerequisites (and for the various reasons I've mentioned, if you're going to use psionics.) It reduces book-keeping across the board... *except* for when inherent bonuses from Tomes and wishes come into play. (But since those effects are pretty far up the power scale, they can probably be adjudicated on a case-by-case basis.)
Okay, now the extra bonus reason to prefer house rule #2 in a campaign with Psionics: the
psychic reformation power. (
http://www.d20srd.org/srd/psionic/powers/psychicReformation.htm)
This power allows you to go back and revise feat, skill, and power selections (and, I would argue, spell choices for casters who must choose spells known), and is a 4th-level Psion/Wilder power. The cost is 50xp per level (split between caster and recipient, if the receipient is different from the caster), and you have to revise every level in order, and respect the set of prerequisites you would have satisfied at that level. If you allow magic items to impact what feats can be taken, then this can allow a high level character to get feats more powerful than he would have had access to at the original levelling time. If you take house rule #1 from the above, it allows a high level character to choose only high-power feats, even at low levels. (Even though his earlier magic items wouldn't satisfy the requirements, his current magic items do.)
Anyway, this is my take on the issue. To be honest, it's never really come up in a game I've played in or DMd. (We tend to be light on ability damage, and we haven't used a huge amount of Psionics.) I *do* expect that it may come up some time soon, since my next replacement character design for our campaign is a high-level Psion, who has used
psychic reformation to good effect (filling low-level feats with metapsionic feats that would have not been useful at the time they were taken.)
Oh--and I'll note the really big reason for the "you can't lose benefits" thing is that I don't believe most DMs or players will pay enough attention to effects that cause feat benefits to be lost in the first place. If I start whacking some bad guy with
strength of my enemy (2nd-level Psychic Warrior power) or
ray of enfeeblement (1st-level Sor/Wiz spell), I don't think that anybody is actually going to be keeping track of "okay, now his Power Attack feat went away." So, I think that the same grace should apply to players. If you're not going to remember to do it anyway, you might as well make it explicit.