At the same time they will be constantly annoyed that this or that API they've read about somewhere isn't wrapped and they have to PInvoke it like WinForms developers have had to on Windows since day 1.
As an iPhone developer myself (working entirely in native Obj-C), this would be my biggest concern. The iPhone development environment (especially lower-level stuff like graphics and networking) relies pretty heavily on a huge library of pure C code. I'm sure they have an automatic Obj-C to .NET bridge (so you can use Obj-C objects directly in C#), but are they providing the PInvoke "headers" (or providing a code generator for them) to give you access to the C APIs (CoreFoundation, CoreGraphics, CoreNetwork, SQLite, etc.)?
If they're not providing them, that's a dealbreaker for me. Having to get the PInvoke signature right for one or two Win32 functions is a pain-- having to do so for the dozens of C functions I use in a typical iPhone project would be a huge hit to my productivity.
There are other technical issues I'd be concerned about, too (toll-free bridging support, how seamless the integration is between Obj-C and .NET types, what happens if I need to access a C++ library (as in Obj-C++), etc.), but the level of integration they provide between C# and pure C APIs is my main concern.
(disclaimer: I like Objective-C
)