My first attempts with the Windows Phone 7 development tools were disappointing, but with WP 7.1 beta ("Mango") I have managed to build and use the csunits class library without problems.
And this is what the Physical Unit Converter looks like in the Windows Phone emulator!
I have previously created a Silverlight unit converter, which can also be found in the csunits GitHub repository. The Windows Phone Unit Converter uses practically the same code, except that the ComboBoxes for selecting quantity and units have been replaced with the ListPicker control from the WP7 toolkit.
It should be noted that the csunits class library compiled already under WP7, but some of the type reflection methods were not implemented, leading to errors when trying to identify the quantities of the library. This functionality has been a prerequisite for automatically loading the full set of quantities in the unit converter applications. Luckily, the required type reflection methods are apparently implemented in "Mango".
This has primarily been a "proof-of-concept" exercise, simply to explore the possibility of developing for the Windows Phone platform. The WP Unit Converter is definitely not in a "production state" right now :-)
I have for the time being ignored some rough edges in the application. In particular, the quantity list picker does not properly display the actual quantities in selection mode. The problem does not apply to the unit list picker controls, so I am fairly confident that the issue with the quantity list picker can be fairly easily solved.
One might ask whether these efforts are easily transferable to iPhone and Android phones as well? In principle I believe the csunits class library would be possible to build also using MonoTouch and Mono for Android, and from there it should be fairly easy to design user interfaces specific for each platform. But I'll leave this question open for now...