I've just tried to run the the install for Office 2010 Beta Home and Business on Server 2008 R2, and I get an error dialogue saying Office requires Windows 7, Vista SP1, or XP SP3. I've tried running the install in compatability mode for all three with no luck.
ADDENDUM: Initially I chose the "for home, small businesses, and school option", which yields Office Home and Business, which won't install on Server 2008 R2. Choosing the "for enterprise and mid-sized businesses" yielded Office Professional Plus, which duly installed without issues. The two intial options are slightly misleading, unless you read the fineprint, which actually includes "IT professionals" and developers in the Professional Plus package.