Version 126.96.36.199 of the basic WP7 project template has been released. The core feature of this release is a new item template that allows you to add a F# Source File in WP7 projects. Prior to this release, attempting to add a new item through the standard process resulted only in options targeting C#.
To take advantage of this new feature, do the following:
1. Install the 188.8.131.52 version of the project template (see this blog post for more information).
2. Right click on the F# project and select Add -> New Item...
3. The resulting window will now include a F# Source File item as shown below: