Can I port my old C# desktop application on IPad using MonoTouch?
You can probably re-use any modular logic.
The MonoTouch UI code is completely different than WinForms (or anything else in .NET really) so re-use there is pretty much impossible.
If your application mixed UI and Logic code in the same place, you'll have to pull out the logic and create separate assemblies for them. You can then build your UI (using InterfaceBuilder) and then wire everything up with some new UI code.
Not if you used any features from the .net framework. From the mono touch website:
When developers create iPhone and iPad applications using MonoTouch, they use the C# language to take advantage of the features found in .NET like garbage collection, full type safety, and Language Integrated Query while interfacing directly with iPhone OS libraries, including UIKIt, CoreGraphics, CoreAnimation, GameKit, AudioToolbox, AVFoundation and many more.