According to Diablo 3's new community manager Bashiok, players will be allowed to respec their characters. While perhaps not as liberal as WoW, players will not be 'locked in' to character choices as they are in Diablo 2. Read the announcement:

There will almost definitely be some sort of system to respec; however, it isn't likely to be as liberal as World of Warcraft. We don't want to lock a player into a system that punishes them for mistakes, experimenting, or lack of knowledge early on in the game. We also don't think a system that allows immediate, complete, and at-a-whim changes to a character spec matches the feel of Diablo. It's likely to be somewhere in between.

That said we still feel like the desire to play the same class again that you may have played before is still a part of the game, and with some ability to respec could potentially require other incentives.

Via Gaming News.