Please remember details next time. ie version of wordpress, where specifcally this is occuring within your blog? What specifically you are doing to get this error? etc.
If it’s for the blog linked to in your profile, you’re running version 2.0.1 of wordpress. That function started with version 2.1 and is not supported in older versions of the software. You need to upgrade your wordpress install.
Probably would be a good idea to upgrade as you’ve just told folks you’re running an unsecure version of the software. Like walking around with a “Kick me” sign on teh front of your tshirt, not the back.
