OK, next question: Will the users only be moving from one page to another, within the Tabbed Control, or can they move off of the Tabbed Control all together and go to a control that doesn't reside on any of them?
If the former this can probably be done, but if the latter is the case I can think of no real way to tell when they leave the it, as Tabbed Controls don't have LostFocus or Exit events. If this is the case I suspect you'd have to use standard validation code in the Form_BeforeUpdate event to check for Nulls in the controls. This, of course, would only occur when the record is being saved, not actually when moving from page to page.