I actually just asked this question on car-seat.org the other day in preparation for our upcoming trip, to be sure that if dd was in a booster (unlikely- unless something really drastic happens in the next month, she will be in her car seat) we were still okay LEGALLY. The person who answered my question is one of the board moderators and a certified CPST. This is a snippet of the reply she wrote:
"The very first thing I want to say is that state laws can vary, but the laws of physics are the same no matter what state you are in. A crash is a crash, and crash forces will be the same regardless of what state law says is okay. State laws are the bare minimums, arbitrary rules set by politicians, and they very rarely actually reflect what is safest for a child.
California law says a child must be in an appropriate, properly used restraint until 6 years old or 60 pounds. This means the restraint must be used in accordance to the manufacturer's instructions. This means that some boosters can be used by children weighing just 30 pounds.
However, I must say again, state laws are the bare minimum. They are to be looked at as the floor, not he ceiling. And what parent wants to do the bare minimum for their child's safety?

"
I'm just putting that out there as the response I got from a CPST to basically the same question. It helped solidify my choice to make sure that if DD has outgrown her current car seat (height-wise) that I would buy her another car seat (the Graco Nautilus, FWIW) and keep her in it until she outgrows that. I double-checked her height the other day, though, and she is okay for a little while longer.