Tomboy actually works perfectly well in the Xfce4 desktop environment, so gnome-panel and its transitive deps are superfluous if you use xfce. I don't know if there's a way to specify "either/or" deps in an ebuild without using virtuals, but ideally the gnome-panel dep should be changed to gnome-panel OR xfce4-panel.
The lack of a correct dependency on gnomevfs-sharp was a confusing factor for me as it failed on config with an error about not being able to find the correct dependency. Not sure if this is relevant here, will open a new bug if it is not. It worked after I updated gnomevfs-sharp and gnome-sharp manually.
there is: || ( dep1 dep2 ) what about other gnome-related deps? Unfortunately I don't have any xfce box to test it.
This bug is bogus. To use Gnome applets in XFCE, you need xfce-extra/xfce4-xfapplet, which guess what - depends on gnome-panel.