WordPress 2.1 is incompatible with the previous version of Multiply.
I've already had some mail from people who upgraded WordPress and found no joy, but the plugin has now been updated and, I hope, fixed. That said, there are some things that prospective upgraders should be aware of.
Everything seems to be working on my local install, but I have not had time for proper testing, and won't for the forseeable future. I won't be available at all until Monday at the earliest: if you try this upgrade, you're on your own. (Back up first!)
If you're game, install the latest version of the plugin and follow the instructions on upgrading.