Well, Aperture 1 was released back in November, 2005. Aperture 2 was released a little more than a year ago, in February 2008. If that timeline is at all indicative of their update schedule, then you will be more than fine to buy 2 now. Apple's Pro software like that doesn't see nearly as many updates as, say, even OSX itself.
I would say now is a great time to buy. As you said though, you can never be 100% sure, so the decision is up to you.
One thing to consider is that there is a remote possibility that they will move to Aperture 3 some time after the release of Snow Leopard to optimize it for the use of Grand Central and OpenCL and such, but again, no one really knows.