stick an @ in front of the "Hello, Mono+WinForms!" and see if it still happens.
Works OK on opensuse 11.0, mono 2.0.1.
Please, edit your question and put the mono version you are using.
Standard debugging advice: Start making small, controlled changes, and see what happens. This will help narrow down what the problem is.
Try removing the symbols: , + !
Try removing the space.
Try a variety of shorter strings, and possibly some longer strings.
Once you have a better idea of what the MessageBox will and will not print, you can start to debug that specific problem, instead of trying to debug, "It doesn't work!"
Finally I have found a workaround. This seems to be a bug in Mono related to font rendering. It happens when "Full" hinting is turned on. I usually have it that way. Changing it to "Slight" or "Medium" in System->Preferences->Appearance->Fonts->Details
fixes the problem. Thanks for the help!
When you updated your packages and source code, did you update/compile libgdiplus? We have seen some funky graphical problems if the System.Drawing and libgdiplus versions get out of sync.