This db is several terabytes in size.
Here's what the manual says about the ideal way to upgrade:
2.12.1. Upgrading from MySQL 5.0 to 5.1
[snip] MySQL recommends that you dump and reload your tables from any previous version to upgrade to 5.1.
Arghhhh!!!!! Dumping and reloading a multi-terabyte db would take weeks with mysql!!! That's the best option for upgrading a mysql db? Unacceptable!!!!!!