Migrating to the SD card only migrates the *content* of the Library, nothing else (as far as I know). So, in theory if you don't have the SD card plugged in you would see an empty Library (or possibly some error message). The SD card should be hotswap-capable by using the "Dismount" long-tap popup menu, so I wouldn't expect a reboot to be necessary. Also, the migrate function does have the option of reversal, but you need to make sure your Library will fit within the 3 GB space available internally.

However, it would be great if someone from enTourage could give some definitive information here. I have 32 GB of music on my SD card so I can't migrate to the SD card to try any of this out...