I'm not sure if this is relevant to the Mono part, but I just found out that the License page isn't up-to-date (yet). The now-available iPhone 1.6 does support .NET 2.1:
It's approximately .NET 1.1, as stated on the license page, not Mono. Unity (non-iPhone) currently uses Mono 1.2.5, which is more or less .NET 2.0; not sure offhand what Mono version Unity iPhone uses.