It's not "natural" in the sense that it's kind of a hack with a resistive screen. The N900's touch-panel can only technically detect one point of touch. I believe two fingers are essentially "averaged" (imprecise, but I can't remember exactly how it works off the top of my head). If I am remembering the demo you're mentioning correctly, I believe it uses the position of the first finger (known, since it's the only point at the beginning of the gesture), and then the "averaged"/whatever point during the gesture, to get an idea of the gesture's shape. What you end up getting is essentially an origin point, and a vector taken from it. It's doable, but it's kind of a hack, as I said.
Note: if I'm getting the technical details wrong, someone please correct me

Update: in regards to the previous post (which was posted while I was writing this), it would be neat, but I can't see Nokia officially pushing a workaround like this into the code of any of their big apps (browser, Ovi Maps), and for those that aren't Open Source, nobody else can, either, of course. I think it's doomed to stay as a tech demo, if only because it would cause mass confusion with newbies suddenly thinking it actually was multitouch, when in fact only a certain gesture performed a certain way "works".

EDIT: as per roger_27's post, I'm referring to the type of resistive panel found in the N900; I am aware that resistive touch technology in general is not incapable of multitouch.