Rejoice, Windows Phone 7 users! You are now free from the cloying constraints of the Microsoft App Marketplace. The ChevronWP7 unlocker allows the sideloading of applications that are not approved for the Marketplace. The developers claim that the pricess is simple enough for anyone to do, and 100% reversible. Windows XP SP2 is the minimum supported OS.
Windows Phone 7 may be an even more closed ecosystem than iOS. Microsoft appears to want to use it as a way to get people to bridge all of their Windows devices together. It's too early to tell if this venture will succeed- early reports have been mixed. The Marketplace has increased to an impressive 3,000 apps, but Redmond still refuses to release any sales numbers. That doesn't necessarily mean Bad Things, but the fact that Microsoft touted the Kinect's million sales so loudly makes me wonder.
Anyway, Chevron WP7 isn't quite like a jailbreaking application. What it actually does is trick your phone into believing it has been unlocked as part of the Developer Registration process. The creators bill this as a way to allow would-be developers to get right into testing and building apps, no approval process necessary. With any luck, Microsoft will be clever enough to see the logic and approach this app with benign neglect.
Chevron WP7 will also allow any user to sideload homebrew applications. Not every denied app is crappy or derivative. Microsoft also blocks apps that run in the background or use the hardware search button. They also ban apps that require direct access to hardware, making LED flashlight apps only available from a homebrew source.
Even if Microsoft decides to get nasty, they won't kill the "homebrew" movement. You can no more stop geeks from fiddling with their toys than you can stop the world from turning or the tide from rolling in. The question is, how long will it take Steve Ballmer to learn that lesson?