Check out this page

http://www.petswelcome.com/milkbone/map.html

You can search Portland and it brings up a pretty long list of hotels, including a La Quinta Inn right near the convention center.

The website recommends calling the hotels to double-check on their pet policies.

Oh, I just noticed they have a database for larger-dog friendly accomodations. Fewer listings, but it's a start.

http://www.petswelcome.com/milkbone/dogmap.html