Well, it seems that Epic is finally confident enough in the UT2003 demo to release it in the next few days. According to this post (thanks H-52) by Mark Rein we can expect the demo "in the next few days":

The demo is nearly ready to go. It should be out sometime within the next few days. Keep watching this space (the stickies) for an announcement. When it's ready we'll just be releasing it randomly to a few major sites so that hopefully it will spread like wildfire and everyone will be able to get it from their favorite games site. There won't be any special exclusives or anything like that. We want everyone to be enjoying it the minute it goes live. More details to follow, keep watching this space.

update: A follow-up post to clarify things.. or well, to make it less clear I guess kinda:

I'm expecting the demo to be out sooner than 5 days but things have a way of cropping up and biting us in the ass so you never know. It could be as early as tomorrow or as late as the weekend but I'm pretty confident that it's this week.

Speaking of this stuff, the Mac port is confirmed. (Thanks OpenGL128)