Confirmed by Major Nelson, Grand Theft Auto IV is now the #1 game on XBOX Live...
http://majornelson.com/archive/2008/05/06/live-activity-for-week-of-4-28-gta-is-1.aspx
Microsoft doesn't provide specific details other than the top 10 ranking. Given the 1337pwn.com trending of GTA IV popularity and quality of the multiplayer gameplay, it may hold on to that top spot for a while.
