Rafael Rivera, Chris Walsh and Long Zheng have put their efforts together to build a simple program that will let you install applications that did not make it to Microsoft MarketPlace. In other words, the software unlocks your Windows Phone 7 and the process is reversible. So it's not like you're jailbreaking your phone. Installing the unlocker on your phone does not modify the operating system.
Check out www.chevronwp7.com for more details.