Dont ask how i found it, but its from a trustworthy source.

It just about confirms that the Downloadable content for the Xbox 360 will be another city 0_o :P:P:P:P

Maybe San Andreas, maybe Vice City, maybe even London

Heres the link

http://www.playthree.net/2008/04/cvg-gta-iv-xbox-dlc-will-be-bigger-than.html