I'll talk about this more very soon, but the short version is the save games will not be fully compatible between major releases.
It won't block you from loading it in the future alphas, but because of the changes I'm sure to make to areas, dialogs, etc - your saved state sometimes won't transition well to the new version in most cases.
You'll be able to export/import your character, though (NWN style). So when I add a new area to the world, you'll be able to just import your high level character and go there and do the local quests, run the dungeons, etc.
Once the game hits the final release, you'll have to start over, though (characters and all).