I was wrong, and in retrospect I should have dug further at the time. There *is* in fact a length requirement - infant seats are only safe if the child is less than 29 inches long. Gwen hit 30 inches on January 22nd. This fact was only brought to my attention in the most coincidental way, chatting with other moms and hearing one mention that she needed to upgrade her son's carseat due to his length.
So technically, Gwen had been "unsafe" in her carseat for about four weeks, without me knowing. Once I did know, I got all snaky about taking her out, and wanted to buy a carseat that exact day. I forget why that didn't happen, but it probably had something to do with having a whole bunch of daycares to visit and interview.
In any case, the new seat was obtained (on Tuesday), installed (on Wednesday), and checked by a carseat inspection agent (today). All is now well. We are very grateful to my grandpa George and grandma Rena for giving us the money to buy this seat way back when Gwen was born. It's a pretty skookum seat, the kind that converts from rear-facing to front-facing (once she is a year) and then a booster seat (which she will use for many years).
The bonus is that after putting the giant hulking Gwen into the teeny tiny infant seat for a few weeks, now when we put her in the Big Girl seat she once again looks like a little baby. Well, a bit, anyway.
First time in infant carseat - 3 hours old
Last time in infant carseat - 9.75 months old
First time in convertible seat - 9.75 months old
She seems to really be enjoying the new seat. I think she is much higher up, so she can more easily see out the windows, and she has even discovered the mirror that we've placed on the headrest of the backseat: it's there so that we can see her face by looking in the rear-view mirror, but if it serves to entertain her as we drive, that's pretty cool too.
Thanks again, Great Grandma and Grandpa!