Will this library work in applications that run on all three devices.  I would like to use the same MVVM solution for all three UI's, so support for all three is important. 

Also, how does using this library affect being able to use MonoDroid and MonoTouch to port application that use it to Android and iPad devices?

Currently the Windows Phone 8 SDK isn't public yet but once it is I will be making the necessary adjustments to support it. As for the 3rd environment are you referring to .Net? If so I plan on porting it to .Net in the coming months but there isn't current support for it.

As for mondroid and mono touch that's really interesting and I'm going to take look to see how plausible it is to port the framework over.

At this point only Windows store apps are support (C#, C++/CX and HTML/JS) with the plan to support WinPhone when the sdk comes out (soon).

Tentatively when are you planning to release Windows Phone 8 version?

I'm looking at a new release in 2-3 weeks and it will include code support for Windows Phone 8 as well a project template or two.