I LOVE Calibre and am happily using it to convert non-DRM files to read on my Kindle, and it works great. But I also have an old Toshiba e800 running Vade Mecum, the Plucker-reading program for non-Palm OSs, and I wish Calibre-converted PDB files would be recognizable as well. I've tried the obvious changes, such as changing filename.pdb to filename.plucker.pdb (which is how the Gutenberg Plucker files that Vade Mecum *does* recognize are named) but it didn't help. And Vade Mecum also recognized a plain .pdb file from manybooks.net so it looks to me like the filename format isn't the issue.
I'm married to a programmer, and from him, it seems that fixes come in two categories: take a quick look, see a minor change is needed, make change - five minutes, tops. Or take a quick look, don't see anything obvious, spend hours (or days or weeks) tracking down the problem and make major code changes.
I've put up two files for comparison on my website
http://www.steigerfamily.com/ebooks at the end of the short listing - the "Problem Files" line with two versions of "A Shropshire Lad" from gutenberg.org (and I tested to make sure they would download using Firefox - hopefully there is nothing tricky that would mess up other browsers). I realize the demand for this type of conversion is probably pretty slim and not worth a major effort. But if Koval or one of his helper angels could take a quick look to see if by some chance it would be a quick fix, that would sure be wonderful.