It looks like T-Mobile is making it very expensive for current iPhone users in Germany to upgrade to the long awaited iPhone 3G. German Wirtschafts Woche reports that iPhone users need to buy themselves out of the current contract first.

For each month left on the current iPhone contract they have to pay 15 Euro (~$23). This only applies to contracts that are already running for at least 6 month.
The contracts are running for 24 month so if your iPhone contract is 6 months old you pay 270 Euro (~$428) plus the fee for the new iPhone 3G depending on which plan you choose.
The financially better choice is to hand down the iPhone to a family member.
AT&T in the U.S. is much fairer to existing iPhone customers. They can just buy the new iPhone 3G for the $199 price and upgrade to one of the new iPhone 3G plans. More iPhone 3G News.