For what it's worth, my 2004-era Nikon D70 and my 2008-era Nikon D700 have roughly identical pixel sizes (with the D700 having twice the sensor size). The difference in quality (pixel noise, etc.) is night and day. The D70 at ISO 800 is worse than the D700 at ISO 3200. My six month old Fuji X-Pro 1, with 2.5x the pixels of the D70, crammed into the same space, has comparable quality to the D700, maybe slightly better even.

Moral of the story: pixel pitch, across cameras built around the same time, may well say something useful about image quality. Pixel pitch across multiple years doesn't tell you anything. And, of course, it's an entirely separate issue of keeping your camera steady and using quality glass to make sure that all those pixels have something useful in them.

(DPreview used to nicely tell you the pixel pitch of each camera next to the megapixel number, but this seems to be gone now. Quite a shame.)