It has been years and things have changed a lot since this was a concern for me but today that have better trailers than they did when I hauled my son in one. we had some super trick car seat designed by GM that looked like a survival pod from some sci-fi movie. I built a trailer to fit the car seat. Today you just need a kid trailer and a floorboard and you can mount many good infant carriers. Here is just one example.
http://www.amazon.com/Maxi-Cosi-Mico...nfant+car+seat
The reason I suggest adding a floorboard is because most modern car seats need something to mount to. A cloth bottom just doesn't work. Still I doubt my wife would let me take an infant on a bike even in a trailer under 6 months old. Cold and wet weather can be another topic all together. And hot weather needs to be taken into consideration.