As I mentioned here, we had to create a small prototype for a client to run on Windows Mobile 6, preferably using .NET 3.5 etc. What I didn't mention is that the target devices must be taken into account before your development takes place preferably, specifically regarding which version of Windows Mobile they will be using, but also which edition. There are a number of limitations/differences with targeting Windows Mobile 6 Standard. These include not being able to use controls such as Panels, NumericUpDowns, Buttons, TabControls etc. If you do try to use unsupported controls you end up with errors such as...
System.Windows.Forms.Controls._InitInstance(WNT wnt, UInt32 dwStyle)
...and similar. Download and use the correct SDK for what you are trying to do.