It's true you have to re-write the UI code on every platform, but it's still the same language on all platforms, so less of a barrier vs. a total ground-up rewrite. Frankly, I see that as a plus, else you end up with badly cloned iOS-like UIs everywhere.
[0] Should note that I'd happily be using MonoDroid if I had a say in the matter.
I too was worried about the accidentally importing iPhone UI into Android. Any tips on avoiding it?
As I said, you have to write separate UI on each platform, and you use the native toolkits (there's no fancy abstract cross-platform UI kit in Mono), so there's little chance of looking much out of place as long as you follow local conventions. It bewilders me to see native Android apps going out of their way to clone iOS widgets and stylings. Not as common lately though; you see it more in usage of PhoneGap and similar, since there's nothing stopping you from shipping an iOS-like CSS on Android.
PS: In the case of Android, I'd flip through [0], and check out Google's apps on a proper device to get the lay of the land.