I ported a Windows 6/6.5 app over to iPhone using MonoTouch a couple of months back. It was very interesting because of the way it forced me to think differently. My windows mobile app was in reality a desktop app shrunk down to fit a mobile if I was honest. Having to do it the iPhone way with its different UI brought me out of my tunnel vision and taught me that my code layer separation was not as good as I thought it was. There was only one book available (but very good : Mc Clure, Bowling et al) so you had to go and hunt through the forums for examples. I could not get to grips with the UI designer so went back to basics and did everything in code, which was a change. Having got to grips with how the screen navigation works (form to form) the rest was pretty straight forward. Looking back I think I mostly struggled with the provisioning side (was doing an enterprise deployment ang getting the certificates to work was somewhat of a chore). I was also forced to buy some Objective C books to try and get to grips with some of the background concepts as there was no real explanation anywhere of of some concepts. That was interesting but I am not going to take it any further. The C# thing works very well on MonoTouch (The IDE is Ok, not the best, but easily good enough) so do not feel the need to go down the Obj-C route. 40,000 lines of code does seem a bit large, but I am not real pro on this. 1 app really does not give me as good basis to pontificate from, but my initial reaction was to flinch a bit. I was under the impression that most of the framework was ported over save the run time dynamic creation stuff because IOS doesn't handle the JIT compilation stuff. So my impression was good, thoroughly enjoyed the experience and will do it again very shortly I hope. A bit rambling but I hope this helps