A recent blog post by Engine Yard claims that they host Groupon's site right now.
http://www.engineyard.com/blog/2011/groupon-makes-history-in...
http://www.engineyard.com/blog/2011/groupon-makes-history-in...
Also, there is no way that Groupon needs a data center--how can they be spending millions on hosting each month? Something doesn't add up...