Have you tried resending your service books before you go on your trip? Sometimes i've seen improper programming or service books update can hose some features, including wifi. Also the access points you connect to, are you sure you have the right access credentials or keys/passphrases to connect to them? If they are open, are you using the hotspot login to make sure you don't have to enter any credentials in the browser before surfing?
The only other thing , which i dunno if anyone in the forums can confirm is , that it could be a possability , but not sure if T-Mo Germany uses UMA, but you may want to disable that (UMA is the service which allows you to swap between cell network and wifi to make calls)
hopes this helps
Mobile/PDA Support Engineer