The guide allows developers to look up their iOS API calls, and then find their WP7 equivalents. And those references include C# samples, and API info to smooth the translation. In addition to the reference tool there is also a 90-page guide for developers looking to port their creations to Windows Phone 7.
At this point in the game, it would take a miracle for the Marketplace to catch up to Apple's App Store and Google's Android Market. But Microsoft is smart to accept that developers aren't going to put all their efforts into the race's dark horse. Instead, Microsoft is making the porting process as painless as possible, and working towards a larger and more comprehensive Marketplace.
source: Microsoft via Redmond Pie