The height limit is just a guide, the baby outgrows his seat when his head is one inch from the top of the shell.
Any rear-facing seat is outgrown when: 1. the child has less than 1" of car seat above his/her head, OR 2. the child has reached the maximum rear-facing weight limit of the car seat. If an infant seat is outgrown before the child turns 1, a convertible car seat can accommodate the child rear-facing to a taller height and higher weight. The height guidelines are put on the seat by manufacturers to give a rough number of how tall a baby can be to use the seat. Remember, a baby who is all legs can use the seat longer than a baby who has a long torso. The weight limits must be strictly followed! (Source: Car-Safety.org, SBS USA) quoted from this site
http://www.carseatsite.com/babycenter_links.htm
A sling is great for those trips to the supermarket. My son was always much happier in his sling anyway. We loved this one
http://www.kangarookorner.com/k_shop_po ... stom.shtml and it would be great for you UK and Canada folk! We used it well in to toddlerhood, it's great when they get to that stage when they want to grab everything, because you can control them much better.
I can't remember how big he was, and he was preterm anyway, so it doesn't count.