As you may know, Macromedia removed Advanced Security from ColdFusion MX (yea!). In its place is a new system called Roles-Based security. However, advanced security was not completely removed. If you try to write a UDF with the same name as one of the old functions, like isAuthenticated, you will get the following error:
Function isAuthenticated is not supported in ColdFusion MX.
It looks like Macromedia didn't remove the old functions, but instead left special 'hooks' for them to really make it clear that they were removed. This is... well... a bit silly. I've already opened a bug report on this and hopefully it will be fixed in the next release. Another function name you may try to use: isAuthorized. This will be blocked as well.