After news broke that the Vita was confining accounts to a single unit, importers groaned at the loss of Japanese PSN accounts. However, Sony has since clarified that multiple accounts are possible; you simply have to pay for them in memory cards.
Sony is trying their best to push propriety memory cards, and the ability to use multiple accounts is one way to force the issue. Each PSN ID is locked to the card, not the unit. In other words, each card can carry one ID. If a user wants a card to use a different ID over time, a factory reset must be performed.
What is okay to do is spread a single account over multiple memory cards, i.e., if you run out of space on one, that account can be carried over. In other words, importers can officially take hold of the hardware, at least as they long as they can afford the proprietary memory.