Whenever we brought speakers to campus, standard procedure was to put them in the Campus Inn. It's a high-rise hotel and looks pretty nice. It's also maybe 2 blocks from Central Campus, and about 3 blocks to Main St. If you don't want to depend on a car (parking on campus is tricky) then I'd suggest the Campus Inn. The Campus Inn, and the Bell Tower Inn, are the only hotels within walking distance of Central Campus (except for the League, which is part of campus).

The Marriott in Ypsi is seriously less than convenient-- so look for the cluster of hotels on State Street that hrdjeepzj mentioned.

If you would prefer to be on North Campus, the Holiday Inn North Campus is the nicest option up there-- other than the Red Roof Inn. But, you won['t be able to walk to campus. You'd still be a 3 mile drive from North Campus.

There are a number of Ann Arbor FTers, so others are bound to chime in.

Go Blue!!!