I am thinking there is a possibility for a second HDD because with TimeMachine coming out with leopard in October, I expect Apple could use the second HDD to show that they are making a new machine that will effectively use a lot of the features coming out with the new OS. You are right though that it would be a challenge to put it in a "smiller" machine.
Yeah, I think Apple Engineers are pushing the limits as it is in terms of phsycial space usage and heat dissapation in the iMac, let alone of the thing is going to be even more compact now! A second HDD doesn't seem feasible to me, although your suggestion regarding Time Machine does make some sense. On the one hand, why promote such a feature in Leopard if you don't have machines which can natively take advanatge of it? But on the other hand, you'll never see 2 internal HDDs in a MacBook or a MacBook Pro for a long time, and the iMac in many repsects is a non-portable portable
So, I see arguments on btoh sides, but my common sense and Engineering background tell me it probably won't happen.