Alpha by July?

I'm guessing I'm about half done with the "scene" I'm working on now. I'm calling it a scene because it's actually only about five minutes of game play. I'm thinking it'll be done by July. I'm trying to lean toward over-estimating the time needed because I'll probably be coming up with other things I want the engine to do and will need time to add. I'm not sure why I'm setting a date because it doesn't really matter, but I guess it'll be neat to look back after I've finished and see how far off my estimate was.
Anyway, I'm planning on just promoting to alpha status after finishing the scene, since I'd guess by then the project will be in a state usable by other developers, for the most part. After it's in alpha, I'll probably start getting it to work on other platforms. My main goal would be 32-bit Windows and Mac (already runs fine on Xandros and, presumably, Linux in general). After that, 64-bit would be nice but that'll probably be quite a bit of work. I've been assuming (abusing may be more appropriate) 32-bit behavior so far and haven't really given much thought at all to 64-bit.
Anyway, that's the state of things.