A friend of mine has a street "legal" RS 250 here in Vail, CO. He bought it for his wife!!!! He bought a Canadian kit (lights) and registered it in Montana. Couple of my riding buddies had the opportunity to ride it last season and they all, without exception, loved it (and its unicycle ability!!).
I'm not sure exactly what it would take, but I've heard that it's possible to get a streetbike somewhere overseas and import it for street use here. TAKES BIG BUCKS, though... I should think more than would be worth it.
Check out http://www.gpstar.net GP Star imports bikes from overseas, normally for racing purposes. They'd probably know how to sneak one in.
If you're just looking for a 250 for your girlfriend to start on, it would be a hell of a lot easier and cheaper to find her a used 600.
Here's a guy in SoCal who sold his street-legal RS-250. I don't know the details of how he did it, but if you're so inclined I'd be interested in anything you find out... Beautiful bike. http://www.kangol.net/bsone/bsone.html
I have been riding an RS250 streetbike since they came out in 1995. I currently ride a 1998 streetbike and for me it is the best bike I have ever ridden and wouldn't give it up for anything. I don't think that guy got Aprilia parts in Canada as there are no Aprilia dealers their??