Personally I'd ask:
1) List two (significantly) different ways to persist data on a Windows Mobile device. What are the advantages and disadvantages of each one?
2) What problems are you likely to encounter developing for mobile devices that you won't developing for desktops / servers? (Want to hear about limited resources, bandwidth, loss of connectivity and the need for offline working, limited screen real estate)
3) What is missing from the .NET Compact Framework which is in the full .NET framework? (Shows that they're not just a .NET developer blagging it). What would you do when you need to use something that's not included? (Looking for a reference to writing unmanaged code / pinvoke) - tip of the hat to @Mat from his comment.
4) What are the differences between the Windows Mobile development stack and the Windows Phone 7 development stack? (Show's they're staying current)
5) Discuss different ways of communicating with a server from a mobile device. What are the pros and cons of each one.
I'd still also ask about OOP and .NET generally as they both apply, potentially a bit of SQL too, especially if they mention SQL CE in their first answer. Depending on the applications you'll have them writing I'd look at talking about encryption too - both while persisting data on the device and while communicating with the server.