Same thing with my 56 reg Vectra Exclusiv now 4.5 years old and driver's door mirror just fell out in my hand as I closed the car door - unbelievable!!
But intending to stick it back on tomorrow with sillicone sealant - thanks to previous poster oilymechanic for sharing this excellent solution!!
Also intend to check passenger side as above posts suggest it's due to drop out next week.
GB, Blackpool
February 2011