File this under 'rumor' but technically very plausible.
We know Windows Phone 7 will be locked for advanced OS options and privileges and that developers will have access to these. And like the iPhone and Android platforms, presumably after the device is "rooted" or "jailbroken", the modding community can have a real go at the OS.
What we don't know is exactly how much of the 'Metro' UI can be changed or modified, but for a lot of folks, they're hoping that most if not all of it can be replaced.
Justin Angel, a former Microsoft Silverlight Program Manager involved heavily with software development in Windows Phone 7, said recently in a tweet:
OEMs (phone vendors) can replace Metro completely, but they won't. It'll be easy to jailbreak WP7 into another theme though.
OEM get to ship 2 themes (OEMLight and ORMDark). A Theme _can_ customize anything, including control templates.
OEMs will have an advanced tool set not available to the public (how long before those leak) which gives them access, to among other things, the phone-radio and evidently the UI (though one wonders what WP7 would "look like" without 'Metro'?). The good news is something we expect to here more of: once the device is jailbroken (which won't take long), people will be able to modify the UI to their liking.
How deep we can go is all the remains. Hopefully it's more than just changing color "accents".
[via @JustinAngel 1, 2]