ERROR 1018 (HY000): Can't read dir of '.' (errno: 24) and other jazz funk classics

Errno: 24 means 'Too many open files'. Everything you do in linux is a file (or rather a file descriptor).
Every mysql connection, every apache connection etc.

By default ubuntu allows users to open 1024 files. The hard limit is 8192. So if you start to hear the melodies of errno 24 its probably time to start increasing this limit

To see how many files a user has open try this

edit the file in /etc/security/limits.conf and add these two mysql lines in at the bottom

There is some debate as to whether you need to reboot for this to take effect (i say debate, i rebooted, and didnt google for it as i was in a rush ymmv).

Once this is set you need to make some changes in the mysql file as well. Assuming yours lives in /etc/mysql/my.cnf - edit or add these values

The table_cache should be researched (start here: http://dev.mysql.com/doc/refman/5.0/en/table-cache.html) and changes monitored. The default value is 64, so 512 should be a safe bet to begin with.