Microsoft recently revealed a web site (not to be confused with a tool, as it was stated earlier) - API Mapping Tool, that allows developers to inspect their iOS applications and learn the Windows Phone counterparts for most SDK components. It offers class documentation structured in the same way as other MSDN documents. There is a default tree of iOS capabilities, each linked to a Windows Phone alternative with links to documentation from both sides.
Although not the complete API is covered, the most important capabilities, like GPS and Accelerometer sensors, are documented enough for an easy transition. For some of the analogies there are simple code snippets that demonstrate ways to invoke specific phone functions.
The tool is only available online and is currently associated with iOS 4.2. Some capabilities listed in the published docs are not yet available in the Windows Phone OS (e.g. sockets) - those will be released as a part of the Mango OS & SDK update.