Mono 1.1 is the unstable tree of Mono. I just bumped with the 1.0.4-r1 to 1.1.2 in my portage overlay and the compilation worked fine.
Only libgdiplus made trouble. I also tried it the same way with the same version as mono.
checking for pkg-config... /usr/bin/pkg-config
GLIB 2.0 installation OK
configure: error: "Did not find Cairo 0.1.22"
!!! ERROR: dev-dotnet/libgdiplus-1.1.2 failed.
!!! Function econf, Line 449, Exitcode 1
!!! econf failed
!!! If you need support, post the topmost build error, NOT this status message.
foobar ~ # equery list cairo
Searching for package 'cairo' in all categories among:
* installed packages
[I--] [ ~] x11-libs/cairo-0.1.23-r1 (0)
1.1.2 isn't ready for the tree any time soon. The only reason i *may* add a version of the 1.1.x series is for the amd64 folks, since 1.1.x is the only version that even comes close to working for them. Marking LATER.
1.1.4 and higher are in the tree. changing from RESOLVED LATER to CLOSED.