Same problem here. I have the MBP 13" Early 2011. I used a Dynex VGA adapter. It worked great when I first got the adapter in May 2011, but when I tried using it recently in January 2012, the MBP could not recognize the same projector I was using from before. I am on Snow Leopard and have not upgraded to any new OS, however I have installed updates as that they released from time to time.
If there is a known problem, I am all ears. Thanks.