About the only games that do any kind of public beta testing (with the exception of Q3 when it came out) tend to be massively multiplayer games that RELY on multiplayer and to a smaller extent games like Diablo 2 for that same reason.
LucasArts has a history of having closed source, closed beta games, so I wouldn't get my hopes up. Sure, this is Unreal Tech, but then, why would they need extensive external testing? The game isn't going to be huge with hundreds or thousands of players online on one server, so there isn't much reason to, except fanboy hype.
If there is a demo, I would expect (as per gaming industry tradition) that it won't come out until after the game has gone gold.
As to a demo I'd love to see another "it's not just a level ripped from the full game but a new scenario to show off the engine but not spoil everything" type. But that's just a hope, I have nothing to confirm or deny that this will happen!
PS: I'm sure you guys all heard that RC is being delayed (until Fall/Winter 2004).