i think limited production is hardly a reason for retaining good resale value. if the demand drops after 1-2yrs (like what happened with 350z), it won't matter if there is only a "limited" number of 370z around. In fact, I can see people who were originally interested in 370z opting for a used cayman s a few years later since price gap won't be as huge. and if i'm not mistaken, AMG always has a limited production, but their resale values usually plummet after 1-2years.
i think Z is a relatively good "investment" now because it's a big bang for the buck. Worst case scenario you lose 20-25k (guesstimates) after 4 years. not too bad considering the 2008 M3's probably dropped that much already in this crappy economy. if only i could afford a z right now...