I was recently installing mysql 5.0 on a RHEL 4 box and was getting the following really weird error:

[root@mysql mysql]# service mysql start
Starting MySQL ERROR! Couldn't find MySQL manager or server

After looking at all the install documents and making sure I did nothing wrong and that mysql 5.1 did not just need some weird install I found a bug report:

http://bugs.mysql.com/bug.php?id=18820

The last post gives a very simple solution of just commenting out the basedir= and it will start fine, which it does. Once you fix the configuration file the mysqld will work normally, this is apparently just a bug/problem in the current rpms for rhel4 and not in the source or any other release.