About your germany trip question: I have no idea, sadly, because so far I don't even own an iPad.
However, what happens if you swap your ATT sim for a t-mobile one? (Aside from the fact that since this week, they are one and the same..)
Do you have to 'unlock' it first via iTunes?
I can only tell you about the iPhone 4.. I witnessed a young girl with her mom buying an iPhone 4 and they said they don't have a computer, so the saleslady hooked it up to their computer and she said "No problem, I'll hook it up to ours to unlock it." and that's exactly what she did. So I'd think they might be able to help you with the iPad as well, unless you really end up in a supermarket selling simcards.
I suppose it all depends on how much money it's worth to you to have 3g for those two weeks.
If you don't need it right at the airport and money is no issue but you'd like to possibly have the aforementioned kind of support, you should check out a mall in the frankfurt area, because those most definetively hold a multitude of carrier-owned stores. A t-mobile store or an o2 one, vodafone or BASE, and I guess they'd be the safest bets. The ones I provided in the article-links are the cheaper, but I doubt you'd get support like this there.
For t-mobile, you might want to check out what they call: Xtra Web'n'walk Flat Special -- 34,95 for an entire month, unlimited traffic slowed down after exceeding 3 GB, includes a HotSpot Flatrate, which for example means you get to use your ipad with Wifi speeds in McDonalds

Simyo might be cheaper, but no idea about how to acquire their simcards.