The page tells that Unity iPhone supports .NET 1.1 version.
As I know, a version of mono and original windows platforms .NET are different.
Due to after windows is released then mono's is came.
So usually mono's is less: mono 1.1 is most compatible to .NET 2.0 of windows.
If so, the version .NET 1.1 which Unity iPhone support means mono's one or windows one?
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:
iPhone 1.6 Now Available
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.