A while back, someone discovered a file on the PSP that was the same as the one revealing a second generation PSP (2000 Series), but was meant for a third generation PSP.
Tonight, I was looking at the PSP manual online, and saw the 2000 Series graphic used on Sony's site. I went to the image, replaced it with a 3, and there it is. 3000 Series.
1000 Series is there too, but no number after 3 shows up; any subsequent numbers give a 404 error.
I think this pretty much all but confirms there is definitely a third generation model coming.