I am running some simple code which works on .Net with no problem.
MessageBox.Show(Clipboard.GetText());
Clipboard.SetText("hey man this is a test, okay?\nand it's not bad.");
MessageBox.Show(Clipboard.GetText());
On Mono on OSX (Mono JIT compiler version 2.4.2.2 (tarball Wed Jul 15 13:30:27 MDT 2009)) it does NOT interact with the System clipboard. I imagine that this is impossible... on OSX? On Linux? On Windows?