Encountered an issue today with a theme for wordpress 3.0. The error arrived when i tried to goto theme options.
How to fix it? well simple answer is that there was no theme function / page defined with that name. Have a closer look at the url that gives that error and then try to find out the similar words in the functions.php of that theme.
Here you can see an example of how i fixed this issue in the theme that i got error on today: