Any firearm cambered in 5.7X28mm isnt avaliable to civilians in the US because the cartridge was designed to defeat level IIIA body armour - obviously a plus if you are crime wanting to do battle with the law.
For technical info on the P90 have a look at this site :
http://www.remtek.com/arms/fn/p90/index.htm
(It would still be fun to use one though!!)