Clean URLs related troubleshooting

I have just found the solution to an old issue I first reported a year and half ago: http://drupal.org/node/104688. I first suggested a change to the core code to solve it, but it turned out the problem was in the RewriteRule the whole time.

Since there were some other users who ran into the same problem and who might benefit from documenting the solution, I'd suggest adding instructions on how to correctly write a RewriteRule needed for Clean URLs. The page that seemed most relevant for this is http://drupal.org/node/256410.

I believe it should contain a detailed explanation of what the RewriteRule and the two preceding RewriteConds do, and why they are all necessary - in particular, why the 'QSA' flag is needed, since that was what I was missing. It was missing from the Drupal 4.7 default .htaccess as well, which is how I first encountered the problem.

Any comments? I could write this myself, as I now have first hand information about the issue.