I don't see them trying to trick anyone, but rather trying to stay on the cutting edge. Just do your due diligence and you'll be fine. Granted that with the 8600 vs. the 8800 series, the difference in performance was like night and day. Now with ATI pushing Nvidia hard to come up with better GPU solutions, the difference in performance with the 9xxx series vs. the "great" 8800 (G92) series isn't as remarkable, but by the end of the line, I'm pretty sure you will see a huge improvement as before. I think history will repeat itself, so be looking for a mid-term shift once again with the 9800 series, including the addition of a new and improved chip architecture, G96, etc...
For me, it all makes sense in the end and I can't blame Nvidia for trying to earn a profit and stay ahead in the GPU game. If they were sitting around now, resting on their laurels, people would complain about that too. They just can't seem to please everyone, so they just decide to keep making a profit, keeping their shareholders happy.
