Ice dam in the making
 

Had an early snow here yesterday/last night, here's a pic of my roof:

http://img32.imageshack.us/img32/4497/img3564vl.jpg

Was up in the attic a few days ago air sealing the leaks so I know the insulation there is fine... looks to be coming exclusively from the bathroom fan (which ran all night last night, just painted the bathroom and was trying to vent the fumes)

Supposed to have above-freezing weather for the next week so it should all melt off, but this is bound to happen again once it gets cold and I'll have a bit chunk of ice hanging off the roof. Downspout is just to the left of the picture, will probably freeze solid too.

What can I do about this?

You wouldn't normally have the bathroom fan running continuoisly, pumping warm air onto the roof and may not be a problem in normal conditions. You could look at a raised vent to move the air away from the roof.