iphonesim
I know this works the other way around because I have my old 5 here in Germany right now and removing my U.S. sim card and putting in a pre-paid German sim card worked just fine.
I'm due for an upgrade and my phone is basically on its way out. I'm considering buying a new iPhone 6 here in Germany. When I'm back in the U.S., if I put my old U.S. (verizon) sim card in, will the phone work as usual?
This is, unfortunately, expected behavior. Whenever you activate an iPhone (which you do as part of an OS install) it talks to Apple's servers and does various things like exchange crypto certs for APNS, etc. It also checks the IMEI of the phone to see what carrier Apple sold it to, and validates the SIM is valid for that carrier (if you buy an unlocked iPhone in a country where you can buy either locked or unlocked phones the vendor will actually copy down the IMEI number of a normal locked phone, and then enter in their system where it gets pushed to Apple's servers as an unlocked phone, there is no physical or configuration difference on the phones).
The easiest way to get it activated is (as you surmised) to use an AT&T SIM. You mentioned you have an iPhone 4, you can activate it with that SIM. There are plenty of companies that sell microSIM to SIM adapters, but if you want to do it right now you can just place the micro SIM directly in the iPhones 3GS's SIM try with tape. I would not recommend doing this on a device you intend to keep the SIM in for a an extended period because the SIM is likely to come loose (and jam the SIM tray), but if you are just inserting it, activating it, and removing it you should be fine so long as you are pretty careful with how you apply the tape and not shaking the phone too much.
From the Apple web site:
The unlocked iPhone includes all the features of iPhone but without a wireless contract commitment. You can activate and use iPhone on the supported GSM wireless network of your choice, such as AT&T in the United States. The unlocked iPhone 4, iPhone 4S and iPhone 5 will not work with CDMA-based carriers such as Verizon Wireless or Sprint.
So the genius led you astray.
Best Answer
As long as the phone you buy is factory unlocked (not a "no-contract" carrier phone), it should work. Same goes for phones bought in the United States: as long as it is unlocked, there shouldn't be a problem
One thing to keep in mind, is whether the models sold in Germany support LTE networks in the U.S. and on which carriers the phone can be used. It's most likely that the only major carriers you'll be able to use in the U.S. will be AT&T, Verizon and T-Mobile, among a few smaller carrier. See this link from Apple that explained which models can be used on which carriers.