Basically, you still need a networked mac to use as a build machine. Also have to use Interface Builder on the mac to design XIB UI files - that's a great shame and hopefully the smart folks at Xamarin will figure that out eventually. I would LOVE to get rid of that step - everytime I fire up XCode/Interface builder, I want to throw my mac through the window...!
Does the need for Mac apply even if you use Xamarin Studio?
BTW, I know what you mean about IB. The way you have to hook up controls to their event handlers is one of most retarded and unintuitive development paradigms I can think of.