On my commute there is a 3 mile section where my only option is a shoulderless road that is VERY busy with a 45 mph speed limit. For 2 miles of it I take the sidewalk. Peds are rarely on it, and I go slow around them, and I don't have an alternate route. I think sidewalks are fine as long as you keep it slow-ish and stay conscious of things.

Funny story to add, too: The other day I was riding on the sidewalk and there was a ped zombied out on his ipod, taking up the whole path. I checked, no cars coming, so I bunny hopped off the curb behind him into the lane. His ipod obviously wasn't loud because the noise made when I landed made him jump about 5 feet into the ditch on the side of the pathway! I felt pretty bad, but at the same time I was really doing him a favor.

Ride the sidewalk now, ask questions later!