Doh! Thanks for that. I use an Olympus E510 - last year I eventually became aware of barrel distortion in some of my photos taken with the 18-42 at wider-than-standard (<25mm). I managed to adequately correct some of them in the gimp (apply negative lens distortion), but my attempts to fix others didn't look better or worse, just different.
Meanwhile, now I was aware of the problem I took some more pictures, using the older, and bigger, 18-45 from my E500. In those I became aware of the distortion in-the-viewfinder on some pics, which is very disconcerting.
After that, I took a series of pics with all my lenses at various lengths and apertures. Used 'display' from ImageMagick to quickly review these, but I couldn't see _any_ significant distortion. By now, my brain was starting to hurt, and I began to doubt my eyesight (and discarded those test pics).
Now I've read your comment, and compared one pic where I remembered mild barrel distortion - sure enough, the raw (.orf) shows it, but olympus's own jpeg is corrected. Thanks again, I'm off to take some more test pics.