I don't think a new bumper, slightly revised interior or a couple color changes will do much for the 370Z at this point. It's a great car already but as others have said, the competition is catching up. I'm not sure there's an 'average' consumer for the 370Z but it definately attracts a particular following. I'm unique because I never really liked any of the past Z cars so I never bought one, however when the 370Z came out it changed everything.
The economy is rough, supply from Japan is limited and dealers are taking advantage of the situation (at least dealers I've been to...not just Nissan). I do agree you get a LOT for your hard earned $ with the 370Z however you can't overlook the used car market. These days $35k will buy you a LOT of performance car on the used market.
People also seem to be holding onto that 5-7 year old (still mostly shiny) car they purchased back when the economy was hot. Now, this probably has minimal effect on most 370Z customers who are looking for updated performance, but overall I think this would lengthen the product life cycle for car designs as the ROI takes longer to achieve. The 370Z product development was during a time when the economy was fairly stable and the housing market was much more viable. I'm certain the downfall of the housing market and financial industry has blown more than one business plan out of the water for car makers.
Even a very minor change to something like a bumper requires a lot of effort for a large manufacturer. You have product design, prototypes, testing, tooling, manufacturing line changes (either internal or external with suppliers, possible NRE charge with suppliers), component discontinuation, possible scrap inventory and revised marketing to implement.