[Natch]

One way for a basic mod_rewrite of the members section: in your .htaccess, you need the following

RewriteEngine on
Options +FollowSymLinks
RewriteRule ^member([0-9]+).html$ member.php?u=$1 [L]

Now to have it based on the membername, you would need to do a little on-the-fly manipulation of that .htaccess file: this is dangerous, as a bug in your .htaccess can make your site unviewable...

This said, it is doable to have a cached htaccess that is updated via a daily cron job based on the userlist, but it would make for high server overhead having a N-line htaccess file where N in the number of members you have...