We'll have the demo available upon release (as that's mandatory for the XBLA version, so we'll do it for other versions too). Beta is an interesting thought and we have some ideas, but at this point it's hard to say if there will be a beta... We'll have to see closer to release.
Early beta release for fans? Hell yeah! With things that wasn't included into final version of was removed from retail. Some earlier level design, features and such - would be cool. I can live even with the fact it will be ab it unstable because of obvious reasons.
if you have a level editor, it would be awesome to have a beta for it. That way when the game becomes available there will be mods for it. Tripwire is doing that and its a great idea.
Chances are you would have already announced a level/mod editor already if you had one however