Help me unlock a German iPhone 4 iOS 5.1?

I just bought a second iPhone for my sister on the net for almost no money. When I got the iPhone today, I saw why.

It's a German iPhone on iOS 5.1. It's locked and it only accepts the SIM provided by the carrier. It's still on the setup screen (where you have to set the language, locations, etc...)

Since jailbreaking/unlocking iPhone isn't new for me, I tried already a lot.

• I tried downgrading it to 4.3, but according to RedSn0w I can't get him into DFU modus.
• Restoring using iTunes didn't work out either because iTunes is asking for a valid SIM card.
• Selecting a custom ipsw using iTunes didn't work out either. I guess the baseband to high.

I also know it's next to impossible to unlock such a device. But I'm sure someone in this community knows a way around.

I'm prepared to give 50 reputation (whenever I can) to whom can guide me to install the iPhone (so I actually can use it and see the SpringBoard) and another 50 to whom can tell me how to unlock this device.

So any suggestions at all are most welcome!

Here is a picture of the lock-screen:

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.

…

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 "unlock", 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.

In response to the 'sites' unlocking the iPhone- the places that offer it as a service who aren't Apple do it via jailbreaking - the only way to unlock an iPhone that has been classified as locked by Apple servers is by stopping the iPhone 'calling home'.

In terms of unlocking your iPhone, as legally unlocking it isn't an option (above), you may as well jailbreak it yourself, for free.

This website seems to have good instructions - I've followed them before when I jailbroke my old iPhone, but its worth bearing in mind that a) jailbroken phones seem slower and b) the 5.1 jailbreak is tethered.

Last point (in a somewhat disjointed answer) if you want to restore the iPhone sans-SIM, use XCode's device manager.