Apple tends to release new Macs on a somewhat vague cycle (so it would seem), but who knows? I highly doubt that they would release a new Macintosh in conjunction with the new operating system. But if it debuts at Macworld 2009 as rumors suggest, it's possible.
I'm guessing a MacBook refresh is due later this year while the iMacs will not receive any significant updates for awhile (they were just updated with Intel's Penryn-class processors). A MacBook Pro update could very likely join the MacBook update as well, leaving the Air to be updated before or after that, but I would not count on all three notebook lines to be upgraded on one day. Just a hunch. And while I'm listing my upgrade predictions, the Mini needs an update today, but it's as good as gone unless Apple can breathe life back into it like they are doing with .Mac/MobileMe. The Mac Pro might not see any updates at all for several months, as was the case with the original Core 2 Duo Mac Pros and their replacements that arrived a week before Macworld this year.
"Snow Leopard" is going to be more security/stability oriented, so I doubt it will accompany any new machine. This sounds more like an actual Windows Service Pack than the other OS X installments, and when was the last time you saw a new machine introduced exclusively with one of those? Granted some computers would be advertised to have it but it's not the same case exactly. For example, when the Air was released, it was introduced with the new multi-touch trackpad and remote disc/access (etc.) features. The original MacBooks and MacBook Pros were introduced with the integrated iSight camera like the iMac G5s before it... Now things like that aren't exactly OS updates, but you get where I'm going with this.
Buy the iMac now or as soon as possible. And I recommend upgrading the mid-range 20-inch model (the 2.66Ghz model) to meet your needs or going beyond that. The entry-level iMac is nice, but for the price difference, getting a more powerful model would be worth it. Just my two cents on that if you needed it.
The iMacs were just updated and now is the time to get one before they become outdated and replaced again, though like I said, that may not happen for awhile.
-BMF