A recent (at the time) episode of the Simpsons (the first "Treehouse" episode) featured aliens Kang and Kodos, whose original appearances were supposed to have been characters only Homer could see. It seems they transplanted that concept over to Al.
This episode marks the first and ongoing thereafter occurrence where the studio audience cheer Al when he walks through the front door.
This episode was taped on October 19, 1990.