Desdichado
Hero
I'm a fan of the notion that somewhere around +10 or so is the practical limit. One of the reasons I like E6 is that even with crazy specialization, you're not likely to get much beyond that number. You can't get a BAB bonus higher than +6--factor in some STR bonus and maybe a masterwork or magic bonus on your weapon, and you're in the neighborhood of +10. Your good base save is going to be +5. Again, with some ability mods and a cloak of resistance or something, you'll be in the range of +10. For skills, you can get a little higher, since you can get have a rank bonus of +10 already at 6th level. Add to that your ability modifier, any bonuses from any magic, the Skill focus feat, any racial bonuses, etc. you can easily get up to +15 or even a bit higher. But, in general, I think that for skil bonuses the practical limit is a bit softer, and skill checks in general tend to have a slightly less dramatic impact on play than attack rolls and saving throws anyway. Which I think makes a difference in what the practical limit should be, too.
Come to think of it, I think external factors like that make a big difference on practical limits. Another problem with the practical limit on attack rolls is AC. D&D doesn't handle this as well as other games in the d20 family, because it doesn't have a scaling AC or Defense score, like d20 Modern, d20 Star Wars, d20 Wheel of Time, etc. all do. Therefore, attack rolls start to become kind of silly without patches to make the AC scale upwards; either natural armor for monsters, or actual armor for characters. If all the ACs you're going to be attacking were unarmored humanoids, for example, then you'd rarely face an AC higher than 14 or 15 at the top limit, even for higher level characters. In that case, there's really no reason to have an attack bonus of even +10; you're almost in autohit mode when your bonus is that high.
Come to think of it, I think external factors like that make a big difference on practical limits. Another problem with the practical limit on attack rolls is AC. D&D doesn't handle this as well as other games in the d20 family, because it doesn't have a scaling AC or Defense score, like d20 Modern, d20 Star Wars, d20 Wheel of Time, etc. all do. Therefore, attack rolls start to become kind of silly without patches to make the AC scale upwards; either natural armor for monsters, or actual armor for characters. If all the ACs you're going to be attacking were unarmored humanoids, for example, then you'd rarely face an AC higher than 14 or 15 at the top limit, even for higher level characters. In that case, there's really no reason to have an attack bonus of even +10; you're almost in autohit mode when your bonus is that high.