Unauthorised memory units are verböten under the next dashboard update to Xbox Live, which isn't of mainstream concern right now, but will be to those participating in the Xbox Live Preview Program.
Says Major Nelson, the Xbox Live community guru:
When Preview Program members start receiving the Xbox 360 system update next week, one of the changes is that unauthorised Memory Units will no longer work with the Xbox 360. If you've moved your profile or saved games onto one to "back it up," you'd better move it back onto an authorised Xbox 360 storage device prior to taking the update. If you continue to use an unauthorised Memory Unit after the update, you will not be able to access your stored profile or saved games.
If you're interested in what devices are authorised storage devices, that list is about three names long.