Though it’s not yet complete, this is an important development for all users who may want to run code not found on the Windows Phone Marketplace. Healthcliff74 on the XDA-Developers forums has managed to solve the problem of running homebrew code on an unmodified handset. The trick is achieved through loading custom certificates into the Windows Phone 7 certificate store, but the process to achieve this is rather convoluted, so it has not been released yet.
Once the process has been cleaned up, Healthcliff74 promises to integrate the entire process into his WP7 Root Tools unlock app, which should make unlocking your phone for homebrew just as easy as some Android phones.
If you want to know more about the development and progress, you can follow the original XDA-Developers thread here.