Earlier this month, Sony released some information regarding the PS Vita, which reads as follows:
Your PSN ID is bonded to your memory card and your memory card is bonded to your Vita, so if you wanted to change different PSN users but use the same memory card, you would need to go factory reset.
It seems that they are now going back on that, telling Wired and GameInformer the following:
[The] PSN account is tied to the hardware and the memory card, not just the card, which means that if a second person is using your Vita, it’s not just a case of switching out memory cards, it’s clearing out all of your saved data on the Vita itself when you do the factory reset.
Like PSP, PS Vita is intended to be played by only one user as a personal portable entertainment system. As such, the spec is set so that only one PSN account will be connected to one PS Vita hardware.