Recently, WordPress made a MySQL 5.0 database mandatory. I was running 4.0 on WaterBlogged!, so I had to upgrade if I was to keep the WordPress installation current. I don't have much database experience, but it turned out to be very, very easy. Also to note: my site runs at 1&1. Here's some notes for anyone else in the same predicament:
1: Export the ol' 4.0 database via SSH:
mysqldump -h dbxxx.perfora.net -u dboxxx -p xxx dbxxx > backup.sql
2: Import to the 5.0 database (you can create this database via the 1&1 GUI or SSH) via SSH:
mysql -h dbyyy.perfora.net -u dboyyy -p yyy dbyyy < backup.sql
3: Edit your wp-config.php to point to the 5.0 database.
4: Login to your WordPress admin page and perform an automatic upgrade as per usual.
Total site downtime should be less than a minute.