I think it's been covered pretty much in these posts but yes, UT2003 was released and wasn't received very well by fans of the original UT. Possibly it was a rushed release, possibly they tried something that just didn't work. In any case, their solution was then to make UT2004 to replace it.
Eventually they got UT2004 released and it contained most of the UT2003 content but I don't think it has every little bit of UT2003 in it. Some of the weapons were changed slightly and the player models altered. In addition they had more of a focus on returning to what made the original UT great, and it does succeed in the whole classic deathmatch gaming.
As has already been said here, partial refunds were given. Some countries had to send their old UT2003 disc back, and in some just the manual (which I can't really understand unless it's something to do with local laws) and they replaced the game with UT2004.
UT2003 is now no longer available. Once UT2004 was out I think they then pulled UT2003 completely. You cannot get it now, not through retail sellers anyway. If you really want it you could try Ebay and other auction sites, but I don't think it's worth it what with the great UT2004.