U.S. hotels, across most major hotel brands and leading online travel sites, including Orbitz, Lodging.com, One Travel, Reserve Travel, and LastMinuteTravel.com." Update: There's a good article about this and about what various chains are doing to help people figure out what the total cost is here.
If you're more interested in comparing cities than comparing individual hotel prices, StarCite has a new service that sounds pretty nifty: the Meeting Estimator. Again, according to the press release, it can provide average per-person cost comparisons for air and hotels in different cities (and customize the results for meetings that will likely have a lot of drive-in attendees). I haven't checked it out yet, but if the data is good (and there's no reason to think it wouldn't be), this could be a handy planning tool.
Update: Marshall Krantz took Search Party for a test drive and was less than impressed.