I'm currently running tomboy-1.4.2-r1 without having dev-dotnet/gnome-panel-sharp installed and everything is working without problems. I don't see why this should be a hard dep, looks more like it's only needed with USE=applet.
Similar situation for gnome-desktop-sharp, that one seems to be not required at all. It's not referenced in configure.in and tomboy still compiles and runs without it.
+ 15 Mar 2011; Pacho Ramos <pacho@gentoo.org> -tomboy-1.2.1.ebuild, + -files/tomboy-1.2.1-xmllint-validation.patch, tomboy-1.4.2-r1.ebuild: + dev-dotnet/gnome-panel-sharp is only needed with USE 'applet' and + dev-dotnet/gnome-desktop-sharp is not needed at all (bug #358673 by Christoph + Mende). Remove old. +