Updated: Eurogamer has added to this story with an "reliable" figure cited by an analyst of 61,000 which is 16,000 up on the original number. That's a hefty step up although still down on historical launches, but - as stated - you can't really compare them, due to the overall economic gloom and the Vita's challenging characteristics.
According to a report on CVG, the PS Vita sold some 44,500 units on its launch week, which given the crappy economic climate is probably pretty good. The story goes on about PSP and 3DS sales, but if they had launched at this time, they'd probably have done just as well/badly (depending on your point of view).
Anyway, I for one welcome 45,000 odd PS Vita owning buddies and will happily play them all at MotorStorm or WipEout (PSN - Goffeentl). Following Japan's lead expect a miserable run of sales until something comes along to perk sales up. Be it Call of Duty, a price cut or something from left field. Either way, well done Sony for a decent launch.