Well it's not available just yet, but according to guys over at Engadget, they were able to get a hold of the software from iPhoneSIMfree.com and install it successfully. So far they are reporting no problems with updates and connectivity while using it on T-Mobile's network. All of the features work with the exception of visual voicemail - which would be expected.
Since the iPhone is not activated before it leaves the store, customers will be able to sidestep the contract with AT&T and simply insert the SIM card of their current provider.Hmmm, I wonder what AT&T is going to do when sales of the iPhone increase, but signed contracts decrease?
Read more about the installation at engadget.com