I encountered a similar problem, and did the following (as always YMMV):
1. After getting the message in the installer, I tried starting up MySQL anyway, and got a bunch of errors, including some related to ownership. I tried changing the ownership of the data directory (per Marc Liyanage's instructions) but I was getting invalid parameter errors from chown -- the mysql group seemed to be a problem.
2. I found this article on the Apple site:
http://developer.apple.com/internet/opensource/osdb.html
I blew away the existing mysql user, then recreated the mysql user by following the instructions in the article.
3. I then chown'ed the mysql data directory using the new user, and I was able to start up mysql.
My best guess is there was some sort of permissions problem that prevented the installer from applying the correct settings.
Good luck!!