I have this theory behind recent Nvidia activities:
The jump from the 7-series to the 8-series was a big one. When those G80's were fresh on the shelves they went for big money; understandably so since it was a nice big kick in the pants for performance. I read somewhere (can't remember otherwise I'd gladly cite) that Nvidia poured TONS of time and money into R&D'ing the G80 and their gamble paid off. They flat-out slapped ATi in the face the market swung big over to Nvidia. Speaking of ATi, around this time they were aquired my AMD. The 2900XT wasn't much to get excited about so ATi concentrates on mid-low range cards and lets NVidia hold the enthusiest/high-end market.
Nvidia loves this because there is no direct competition for their new line of 3D-super cards so pricing is up to them. This worked until ATi came along with their new super-card. Should Nvidia scramble and develop a new series that's 3x better than the old one? How much time and effort went into the development of the G80? I do not know myself but I would imagine there would be some substantial costs to recoup. With no threat from their single competetor, why not just re-hash the G80 a little and call it the G92? It IS better than the G80 and prices are right so the G92 becomes popular. Nvidia could throw tons of capital into R&D'ing another super-card but why? The G92 is holding its own against the competition. They don't want to and believe me you don't want them to either because imagine what prices would be like.
But now the side-effect. Think of all the people that upgraded to an 8-series and like me, were totally blown away at its capabilities. It was a HUGE jump. Good then but now people expect the 9-series to be another massive leap forward but it's not. So what? So you still can't play the garbage that is Crysis (a 'game' now only popular because of its benchmarking capabilities) at high frames. Believe me, frames or not, you're not missing out on a whole lot.
ATi has finally answered with the 3870x2 and caused a big blowout between the die hard Nvidia'ers and the ATi people. The important thing to remember is at the end of the day, it's close. This as brought us great prices and now a high-end card is in the reach of tons more people than it was a year and a half ago.
Could Nvidia affort to keep throwing piles of money trying to break the insane barrier with every series transition? I don't know...maybe? Could ATi keep up? Maybe? Would it be wise for either company to follow such a steep development cycle? I doubt it.
Just be happy with what you have. If you already have a buff G80 then relish in the fact you get to be that cool for another few months or more. If you have a fondess for ATi, be happy that you can now have high-performance from them.