As for individual characters - I'm not concerned. If anything, I've found it easier to match my actual concept every time I converted an older character to a new edition.
The worry I do have however is more with my books. I love my Incarnum, and Tome of Magic is my favorite WotC book ever. I realize that in 4e there maybe be similar stuff so I can reinterpret those characters, but I like the binder, etc. So my worry is in being able to still use the 3.5 stuff, especially the alternative/weird things that they won't covert to 4e for a long time if ever.
Maybe the solution is to stick with 3.5. *shrug* Not a dig at 4e, just a possible fact for my preferences. How easily I can use Tome of Magic and play a binder (or shadowcaster or incarnate, etc.) is more important to me personally than fixing grappling or wizards using crossbows. The answer to that determines when I convert to 4e. If Tome of Magic is useless in 4e, then 4e is useless to me for now.