Yep, mine fell out 3 weeks back, after the cold weather started to wain.
Driver's side! it wasn't an auto-dimming one.
I bought the aforementioned £8 mirror from ebay (good size match, although not amazing quality), and glued it onto the heating pad with a heat resistant glue I bought from tesco.
It looked like the adhesive had failed between glass and heat pad.
It has been rock solid since. and i just make sure i give it a wiggle every time i clean it (weekly)
Still, a pretty lame failing from BMW!