SQL Server Setup & Upgrade
There can only be one default instance per OS.
Be carefull when installing SQL 2005 on a SQL 2000 machine. It's very easy to upgrade the existing instance without releasing it.
Correct, there is no way to revert to DB's back to SQL 2000. The default instance naming is the same in both products to make upgrade easier. You can always supply a different instance name with the second install to run both editions side by side.
Thanks,
Sam Lester (MSFT)