Jenstar reports at this thread that MSN AdCenter has reopened the open sign up period until tonight at 6PM (PST). So if you missed the first open sign up period you can catch this open sign up period today. To do so, visit http://adcenter.msn.com/ and click the sign up button.
Forum discussion at Search Engine Watch Forums.
Update: Also being discussed at WebmasterWorld and DigitalPoint Forums.
