Obviously if you are doing any audio editing or processing, it makes sense to use the original PCM as a source.
For actual listening, I've seen quite a few members on here who claim they can hear a difference. I can certainly hear the "listening inside an oil drum underwater" type of compression in very low bitrate audio, such as on Youtube videos, but I find any MP3 or AC3 encode with a reasonable bitrate to be transparent. But then, I don't have a hugely expensive hi-fi, and I know my hearing response dips at high frequencies (kids - if you regularly go to nightclubs and value your hearing, wear ear protection, 'k?)
While working on the mono mix, I compared two samples of AC3; one at 448kbps and one at 96kbps. I thought I could percieve a difference - but was that because I knew that sample A was a higher bitrate? (I.e. this wasn't a 'blind' test).
I'm considering doing a blind trial on you lot - it wouldn't "prove" anything because there would be too many variables, but it would be interesting nonetheless. Who's up for that?