We put our little man in the bike trailer (it's a trek branded Chariot) when he was a little mroe than 6 months old. We usually ended up lying him down longway's on the seat and buckling him that way so he could nap, but it wasn't the most secure method.

Chariot has a lot of accessories for carrying smaller children, one is for 3 months and up - which is about when I'd want to start carrying our little ones around:

http://www.chariotcarriers.com/engli...es.php?accID=3

The only thing I really worry about is the how the bumps make an infants head bob around, something that like Chariot support would take care of that problem me thinks.