I've never moved the Library to an SD card, but I always assumed it's reversible because the popup you get when you select "Data storage device" has two options: "Internal Storage" and "SD Card".

Could someone who has moved the Library to the SD card (kenjennings, or even sammieb77) check this and see if you get a different poup now, and/or "Internal Storage" is disabled as an option?

We recently uncovered one KB that had an apparent error (saying the PE only supported SD cards up to 2 GB), and enTourage tends to make changes in updates without always publishing them in the release notes, so it could be since the feature was released last summer that some changes were made (or the original release notes less than accurate)...