It would make no sense to me that in a fast-changing technology world you would hold up products that were ready for months while sales drifted off to competitors offering more advanced products than your current ones, just to wait for software that they don't even need. If iMacs were ready now, the smart marketing strategy would be to release them now with Leopard and advertise them as "Snow-Leopard ready," stating that they'll perform even faster when Snow Leopard comes out. Apple could offer discounts on the upgrade for customers who buy iMacs now, thereby hooking them into the Snow Leopard purchase. If iMacs were ready of course (which they aren't).
Besides, we know that Snow Leopard like all OS releases will have some big issues when it first comes out. If iMacs are released simultaneously, people will identify those issues with the iMacs and they'll get a reputation of being buggy. It will not be good PR for Apple.