Help me unlock a German iPhone 4 iOS 5.1?
Edit 2012-04-22:
As of today you can unlock any jailbroken iPhone:
http://laforeta.blogspot.co.nz/2012/04/how-to-spoof.html

What do you need: A jailbroken iPhone that can be activated in iTunes (that is, not officially blacklisted), a computer with the latest iTunes installed and working internet connection.  You will also need to know the carrier that your iPhone is locked to.
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Your phone will work normally with your SIM card and that SIM card ONLY since we have tricked iTunes to think that ICCID is one of the intended carrier's. The phone can be rebooted and connected to iTunes freely without losing the &quot;unlock&quot;, at least for now.
Since it does not involve emergency numbers or IMSI spoofing it will not have any of the issues associated with SIM interposers. Tested working on iOS 5.01 but should work with tethered 5.1 too.


You could check on the official German T-mobile site at what date you can legally unlock your iPhone. When beyond that date you can perform the whole process online on their site. Then you would restore the iPhone via iTunes and have the SIM-lock removed.
https://t-mobile-simlockportal.arvato-services-mocom.com/pubsilo/Home.aspx
To install the phone you could get a German T-mobile prepaid SIM-card; e.g. from ebay.de - current offers - I think even a de-activated one should work for the installation as only the country-/provider-code is checked.