John,
Interestingly, while redoing my ng. website some of the original files stored on CD's started looking dark to me. Hmm....could CD files start degrading in three years?
Yes, they could have, but it wouldn't result in darker images. You'd see corrupted (as in unreadable) files, and/or images with dropouts or lines running through them.
Or maybe my now three year old Mac screen is perhaps loosing some of its brightness.
If it's a CRT then it's almost certainly lost some brightness. I recalibrate monthly, and every few months I'll see the results of minor CRT aging and have to adjust to compensate. Eventually, you can't compensate enough and it's time for a new monitor.
Or perhaps my own preference has changed.
Also a strong possibility. Whether digital or traditional, the more darkroom work you do, the better you become at making subjective judgements. You're probably pickier (and better) than you were a few years ago.
Scott