I think this snippet from my console log after I tried to reproduce your situation says it all:
Code:
2008-08-27 18:45:56.397 OmniFocus[276] OmniFocus calling process_message in MailAction script
2008-08-27 18:45:57.191 OmniFocus[276] Special character ':' found but not handled!
I filed a bug report with Help->Send Feedback, you may want to do so also.