I upgraded mono to the latest Debian package (1.0.5-2), and it went and broke MonoDevelop.
$ /usr/bin/cli --debug /usr/share/dotnet/monodevelop/bin/MonoDevelop.exe
Creating DefaultWorkbench
(MonoDevelop:9543): Gtk-WARNING **: Error loading icon: Failed to open file
'/home/data/resources/icons/Icons.SharpDevelopIcon': No such file or
directory
I tried using the Ubuntu package (monodevelop_0.5.1-3_i386, one revision higher than the monodevelop_0.5.1-2_i386 provided by Debian), but it does the exact same thing (except the executable is placed in a different structure under /usr/lib/monodevelop, and the /usr/bin/monodevelop script uses mono instead of cli). Doing something silly like copying the /usr/share/dotnet/monodevelop/data directory to /home makes the icon error go away, but MonoDevelop still does not start. I even tried rebuilding the monodevelop package with the new mono install, but it does the same thing. I suspect it's related to this bug.
Update: It has randomly decided to start working again.