I wrote some code that does this... except it's for tomcat =P. Like I'm not ALL that bad with c, and i'd never attempt to write that into apache (and I couldn't find anything that did it online). I'd vote you write something you can cron to create vhost config file entries based on ldap and reload them. I'm assuming there's a way to reload vhost defs without restarting the server in apache. If there's not, you should look at a different server imo 'cause that's sorta necessary.
Just fyi, we're running with everything in ldap and loving it, but it's not for the non-pros out there. We've got a team of several coders and it's taken us almost a year of spare time to config a system that handles dns/rdns/user mgt/mail/web/ftp out of ldap and keeps track of statistics properly. We're not even done yet... We're still working on the control panel for it all and debugging.
If you're interested in our project, pm me. I doubt you would be as there's no way yet to run php on it (since tomcat !love php). If php were someday added, it would be run through a wrapper around apache, which would still be affected by the 'slowness' of tomcat.
So if you're still interested, lmk and once we have a version of it set up that won't cause us and you a ton of headaches we could set you up with it.
-- My software isn't buggy; it develops random features --