
Some quick stats: 15 datacenters hosting 148,357 servers sitting on 17,406 racks consuming 72,500KW of utility power as of the end of January 2008 (as indicated by the bar chart).
On the same page there's also an interesting graph showing the distribution of servers per "property". Live Search is in the clear lead with approximately 75,000 (50%) of the servers pushing out those less-than-desirable search results, followed by Hotmail. The other notable property occupying a large chunk of the servers would be "other" (appropriately named), which one could assume be dedicated to XBOX Live services and the like.
http://www.vgwatchdog.com/?p=35