As of today's Ubuntu 9.10 updates, 2010-02-21, Ubuntu, when resuming from a previous lid closure state, does not re-suspend on any other state unless the password is entered correctly and returns to the desktop.
Basically, if you close the lid, Ubuntu suspends. If you open the lid, Ubuntu resumes to a password prompt. At this point, if you close the lid again without doing anything else, Ubuntu doesn't know how to suspend. If you open the lid again, the password prompt is right there as if it never suspended. Closing the lid again suspends fine. Opening the lid again, repeat this all over again.
As you can see, every other time that your laptop suspends and resumes on a password prompt from a resumed session, Ubuntu fails to suspend.
No fixes yet. Possibly removing the lock screen resume password prompt may get around this interval failure.
J