I will be visiting Portland on a business trip, and will be able to sightsee only at night. Is it safe to leave my hotel located downtown and walk around?
I will assume you mean the Portland in Oregon as opposed to the one in Maine. If this is not the case, this answer does not apply.
However, if the Portland, Oregon location is the one intended, I recommend it highly for touring, even at night. It was our base of operations for a Pacific Northwest vacation we took approximately 3 years ago, and the atmosphere was very friendly. Downtown touring, the parks, the very evolved public transportation system, all were very accomodating, and we were extremely comfortable. I tend to the paranoid, and I never felt the least uncomfortable or unsafe. I can't speak to whether it has changed, but I would probably have noticed if there were any publicity of an increase in crime or danger.
Generally it will be safe, but please be aware that any city can have problems. Be careful in the West Burnside Street area. Also, take a look at PortlandMaps.com for a crime map (it can be filtered by specific crime or area). Also see AOL's "Safe and Dangerous Places in Portland" article for other areas of town that may be less desirable to go to (they also include some safer areas). TripAdvisor also contains some good discussions about crime downtown.