We treat them ourselves successfully.
Yes, it's a carpenter bee. She's laying eggs in there.
You need to spray poison in the hole then plug it up asap... and watch close, they keep trying to come back to the same place.
You can plug it up with caulk or wood putty then paint over it.
Be careful, too: They're not considered "aggressive" but they do deliver quite a painful sting.
PS, here's a bit more 'bout those nasty critters, from Ohio State University:
http://ohioline.osu.edu/hyg-fact/2000/2074.html