I have a Macbook Pro 13' (bought in mid September 2010) connected to a DELL 15' external monitor via mini Display port ->VGA connector and I am experiencing the same problem as reported in previous posts.
What I can add to the discussion is the following:
ℹ I have noticed that since the latest MAC OS update the problem has become more frequent. Prior to the update, the MBP used to hang only after entering standby mode (not sleep mode) for a few hours. After the update, the MPB hangs after approximately 40 minutes (if not earlier).
(ii) I tested my MBP yesterday by connecting it to a different external monitor (Samsung 20' Syncmaster) via mini-display port -> HDMI connector. I have been testing the connection for more than 3 hours, at different resolutions and refresh rates. I had no hanging/freezing at all. Indeed, the testing cannot be held conclusive (3 hours are not quite indicative) but, in comparison with the Dell connection, the HDMI solution worked flawlessly.
(iii) I contacted Apple Care (Italy) earlier this morning. They told me that they are fully aware of the issue and at Apple US engineers are analyzing the issue. No idea, however, on the cause of the glitch nor on the timing to have it solved. They cannot even say whether it is an hardware failure requiring replacement of components or something that can be fixed by way of a software update. Assuming the latter, however, they indicated that a software update solving the issue may be expected by mid of next year.
So, apart from continuing testing the HDMI fallback solution it looks like we shall wait quite a while before we can see this issue solved. I am not particularly happy with this as I use my MBP for business reasons and do I need the dual screen environment working properly.