Congrats on moving to Germany. I'm envious. I've been back a year in the US after a 7 year stint and I miss it very much.
Thanks to the SOFA (Status of Forces Act), the agreement is that for service members what car seats are legal in the US to sell & use are authorized for overseas usage too. However, it can be difficult to get a certain type of car seat just because of shipping issues. So if you decide on one you want for the next stage BEFORE you go, it might be easier. You'll be able to try it out, see what you like, and price it without having to guess. If you're planning on shipping a vehicle over, it might be easier to try out your car with a new car seat this way.
Since you're AF, I bet you're either headed to Ramstein or Spang/Bitburg area. Ramstein is supposed to have its super dooper whopper of a BX completely built this year, but they've had a lot of delays due to weather & vandalism in the last year. It WAS supposed to be completed Feb of last year. Just because the store size is larger, don't count on any more stocking of baby gear. Span/Bit is in the boonies. Nice boonies though. Well suited to visit Benelux, Trier, Mosel, Koln, & N. France, but annoyingly short on much of the usual stock. You have to shop between 1 base & an annex to locate anything. Even then, there's no guarantee.
European car seats are easy to install and I just discovered from another poster there's even a model that rf & ff...
Wavo Carseat
But if you want to try out the Britax line in person, you should do so before leaving the States.
As for flying over -- are you flying commercial to Frankfurt or the no brand airlines to Ramstein directly? Either way, you should have no problems installing your car seat in the aircraft. Being military, regardless of age, your child WILL have his/her own seat. Tickets are issued to every single member of the family.
If you have pets, be sure to reserve spots on the flight as soon as you get your port call. There will be an extra fee for them, but you can deduct any boarding, transportation, and prep costs along with other unreimbursed costs on your 2007 taxes. Just file a 1040 + moving expenses worksheet. It's a deduction that's made without itemizing. Almost everything you spend out of pocket --that the military does not reimburse you for -- (except meals) is allowable UNTIL you get to your new location. Then it's all on you. Even extra or overweight baggage, shipping boxes, rental cars & house cleaning services. Just thought you'd like to know
hth & sorry to blather on as usual.