Mike is right, you can upgrade.. however, couple things to note..
1. If you upgrade from 3.0.x to 3.1.0-b1, you MUST run the upgrader via CLI, ie php ./index.php admin/upgrade
2. If you do a fresh install of 3.1.0-b1, you should be able to upgrade to 3.1.0-b2, and 3.1.0 final via GUI or CLI methods.
This is due to a bug in 3.1.0-b1 that breaks the CLI upgrader.