While we didn't get a release date for Diablo 3 at BlizzCon, lead designer Jay Wilson did tell us that there will indeed be a beta for Diablo 3. At this point, it will be a closed beta, most likely for "Blizzard friends and family," but these betas frequently open up to the public at some point, so keep your fingers crossed. While a game going into beta is no big announcement in itself, we'd like to think that this is the first step on the road to a Diablo 3 beta that will find its way onto our hard drives.
He did also say that Diablo 3 will not support mods, so that ambitiously huge GUI user mod interface that you're thinking about building? It won't be officially supported. But hey, you're a hacker, right? Keep whacking away at it and be a rebel.