While upgrading one of my apps to Mango, I came across a possible issue with one of the new APIs. Part of the app simulates the rolling of dice and uses the ReadingChanged event. With Mango this method is depricated in favour of CurrentValueChanged.
So I decided to upgrade to using Motion rather than the Accelerator sensor. The problem is that the “old” WP7 devices do not have all the sensors that Motion covers so comes up as unsupported. So if your app does not require the full features covered by motion, continue to use the individual sensors. Otherwise your app will not acknowledge any of the sensors covered by Motion (the IsSupported methos) and “older” WP7 devices will not run your app.
John Marshall… Visio MVP Visio.MVPs.org
john Visio MVP 
Would you be able to detect the version of the OS and do both ways? Hey, I’m just throwin out an option, I’m still using the old WP7 OS on my phone.
That would work, but my main point was that if you went pure Mango, you could get bitten.