According to a user over at NeoGAF who attended a pre-launch event for PlayStation Vita in Osaka, the pamphlet he received contains an FAQ about the handheld which states specifically that each Vita will be restricted to using a single PSN account. Switching to another account apparently requires resetting the system software to factory defaults.
Only allowing a single PSN account on the system essentially region locks digital download games and content, although it remains to be seen if the content can be downloaded via PS3 and then transferred to the Vita to get around the restriction. With Sony pushing downloadable games for the Vita, this may be their way to keep people from digitally importing games that come out earlier in other regions, or at lower prices.
This should be considered a rumor for now since no scan of the mentioned pamphlet or confirmation from Sony is available to confirm or deny that this is accurate.
Edit: Sony's Brad Douglas has confirmed this via twitter:
"[The] only difference is that Vita requires you to do a factory reset when logging into a new acct, PSP does not"
No further explanation as to the reason for this has been provided, but we'll keep you posted if we hear more.