Tomboy fails to compile with new gnome-sharp release. The latest Tomboy has fixes for it. "Unstable version 0.11.3 released! New in this release: * Quick fix to build with latest gnome-sharp release." Reproducible: Always
0.12.1 has been released. I was able to build this by copying the 0.11.2 ebuild from the gnome overlay and adding gnome-panel-sharp (ikelos) to the dependencies. However, I already had most of the 2.24 *sharp packages (from the ikelos overlay) thanks to my gnome-do install from that same overlay, so I know this probably won't work for most people without updating the dependencies and adding the updated *sharp packages to Portage.
0.12.1 is in the mono-2 package.mask now.