No its not, at least not here (correct me if i am wrong):
"When is it coming out? Is it free? Will there be a demo?
UT2004 is currently scheduled for a February 2004 release. UT2004 is a stand-alone retail product, but it's only $39.99 ($10 less than UT2003) and Atari is also offering a $10 rebate to people who already purchased UT2003. Mark Rein has confirmed there will indeed be a UT2004 demo, and that this demo will contain Onslaught mode (but not Assault)."
"What will I need to play it?
Here are the official system requirements, obviously this is for Windows 98/ME/2000/XP. Again, it'll probably ship with Linux executables.
Pentium III or AMD Athlon 733MHz processor (Pentium® or AMD 1.0GHz or greater recommended)
128MB RAM (256MB RAM or greater recommended)
3GB HDD space REQUIRED
8X CD-ROM or DVD
Windows® compatible sound card
DirectX® version 6 compliant video card (32-128MB VRAM recommended)
DirectX® version 9 (included on game disc)
Internet (TCP/IP) and LAN (TCP/IP) play supported. Internet play requires a 33.6 kbps or faster modem (broadband recommended)"
Again, no mention of it being a expansion pack or it requiring UT2003 to function.
http://www.planetunreal.com/ut2004/
I woulda bought into this deal too, but i will wait for UT2004 instead.