Verizon, not too long ago, announced that they would be opening their networks to other devices and applications some time next year as soon as they came up with some standards those two items had to meet. AT&T, they are opening their network now, as in right now.

In fact, AT&T's wireless network has been open to other devices and applications from day one; it just wasn't advertised before now. So you can take you custom made hacked-to-hell-and-back breadboard made smart phone in there, and they will either activate it, or give you an activated SIM card for it.

And, you don't have to have a contract either, no contracts necessary, which is convenient since you never know how long that Franken-phone of yours will last. So technically, AT&T is the most open network in the biz right now, and they are also the only major network not part of the OHA, so take that Google! If you decide to whip together your own mobile device, I am pretty sure it has to use a GSM radio, as that's what AT&T’s network is made up of, but chances are you'd be satisfied just being able to activate any GSM device you want on their network, so you could go buy a phone from Alltel or T-Mobile and then have it activated on AT&T's network if you wanted.

Source [yahoo]