I use tomboy a lot (unstable dist) and the recent mono upgrades have broken it for me and a few others. I found this with a bit of googling http://groups.google.com/group/linux.debian.bugs.dist/msg/87179a442b1884c2 which gave me a place to start. This is what I did to get tomboy going.

Tried running tomboy from a terminal and got:
[DEBUG]: NoteManager created with note path "/home/andrewc/.tomboy".
[INFO]: Initializing Mono.Addins
Assembly not found: Mono.Addins.Gui, Version=0.2.0.0, Culture=neutral, PublicKeyToken=0738eb9f132ed756

Read about reinstalling libmono-addins2.0-cil in the email and went looking for the package.
dlocate -L libmono-addins2.0-cil
Package libmono-addins2.0-cil not installed or libmono-addins2.0-cil.list is empty.

apt-cache search mono addin
libmono-addins-gui0.2-cil - GTK# frontend library for Mono.Addins
libmono-addins0.2-cil - addin framework fir extensible CLI applications/libraries

Realised the version is 0.2 not 2.0 - reinstalled both because it made more sense that the gui stuff would be in the gui package.
sudo apt-get install --reinstall libmono-addins0.2-cil libmono-addins-gui0.2-cil

Preparing to replace libmono-addins-gui0.2-cil 0.3~svn.r90520-1 (using .../libmono-addins-gui0.2-cil_0.3~svn.r90520-1_all.deb) ...
Removing libmono-addins-gui0.2-cil from Mono
Unpacking replacement libmono-addins-gui0.2-cil ...
Preparing to replace libmono-addins0.2-cil 0.3~svn.r90520-1 (using .../libmono-addins0.2-cil_0.3~svn.r90520-1_all.deb ) ...
Removing libmono-addins0.2-cil from Mono
Unpacking replacement libmono-addins0.2-cil ...
Setting up libmono-addins0.2-cil (0.3~svn.r90520-1) ...
* Installing 2 assemblies from libmono-addins0.2-cil into Mono
Setting up libmono-addins-gui0.2-cil (0.3~svn.r90520-1) ...
* Installing 1 assembly from libmono-addins-gui0.2-cil into Mono

Tried running tomboy again from a terminal and got a different error
tomboy
[DEBUG]: NoteManager created with note path "/home/andrewc/.tomboy".
[INFO]: Initializing Mono.Addins

Unhandled Exception: System.InvalidOperationException: Extension node not found in path: /Tomboy/ApplicationAddins
at Mono.Addins.ExtensionContext.AddExtensionNodeHandler (System.String path, Mono.Addins.ExtensionNodeEventHandler handler) [0x00000]
at Mono.Addins.AddinManager.AddExtensionNodeHandler (System.String path, Mono.Addins.ExtensionNodeEventHandler handler) [0x00000]
at Tomboy.AddinManager.InitializeMonoAddins () [0x00000]
at Tomboy.AddinManager..ctor (System.String tomboy_conf_dir) [0x00000]
at Tomboy.NoteManager.CreateAddinManager () [0x00000]
at Tomboy.NoteManager..ctor (System.String directory, System.String backup_directory) [0x00000]
at Tomboy.NoteManager..ctor (System.String directory) [0x00000]
at Tomboy.Tomboy.Main (System.String[] args) [0x00000]

Tried again and it worked.