First they announced that only one PSN account could be used per Vita. After that announcement, they backtracked and said that PS Vita users will be able to use multiple PSN accounts by changing the memory cards.
Now, we are back to square one because Sony has just announced that users cannot change the PSN account associated with the device until the device is factory reset, Sony told Wired:
[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. In other words, PlayStation Vita is intended to be played by only one user
I know it’s a bad news, specially for the importers and for those who buy games for different regions but handhelds are usually for the personal use so it won’t affect Vita’s demand in the market.