If you have a bike trailer that accepts a carseat, then why wouldn't you bike with your baby as soon as you'd drive with him?
We had our daughter in the bike trailer at three months. Even though she had pretty good head control by then, we kept the little head support thing on the carseat just in case. She's almost six months now, still loves riding in the trailer. We generally keep the front screen down to shield her from wind, and she wears a hat to help keep the sun off.
I'd be concerned about taking her on an all day trailer trip, because I don't really know how hot it gets inside that thing with the flaps down. But for easy around town errands and stuff? She was fine at 3 months.
I'm sure somebody out there will scold me mercilessly for endangering my child. Fine.