In the last 2 months since the iPhone was released, many people have been trying to hack it so it can work with other mobile carriers. This is useful for people outside the US, as well as people that don’t want to pay a premium for the AT&T plan offered.
1. The iPhoneSIMfree.com team broke the lock with software only. They plan on selling their services to people wishing to unlock their phones.
Read More at engadget.

The iPhoneSIMfree.com unlocked iPhone, on T-Mobile
2. George Hotz, a 17 year old from Glen Rock, NJ broke the lock again. His hack includes hardware modification and isn’t as simple to do. He posted instructions on his blog.
He was interviewed on CNBC, here it is:
3. A group of israeli hackers claim to have unlocked the iPhone, they provide pictures, and will post instructions soon.
Their site (Hebrew)

The Israeli iPhone, on the Cellcom network
