Songsterr is a website that provides music tabs for guitar, bass, drums and vocals and allows you to play back the tabs with extra features such as a metronome for paid subscribers.
They currently only have apps for iOS, but Alex Ross from Zombies Ate My Box that they are planning to expand to other mobile operating systems, specifically Android and then Windows Phone 7.
While they are currently prioritizing the Android app, it is encouraging that Windows Phone 7 is being mentioned in the same breath these days when talking about expanding mobile platforms, even for niche apps.
For those who can not wait, Guitar Tab, currently in Marketplace for $1.49 with a free trial, provides some Songsterr integration, but does not offer all the features that an official app would.
Thanks Alex for the tip.