It seems to me that the Celeron benefits from always being re-released just when Intel has come up with a new chip manufacturing process. You're right, so many of them have been overclocking champs...
I don't think it's worth the risk, though - chip prices are so insanely low. That $30 that you'll need to spend for a monster fan would get you another few hundred megahertz of CPU speed, or whatever... there just isn't a lot of point in putting the strain on your components.
Intel just released a 1.2ghz Celeron, by the way. It's priced at about $130. 100mhz FSB, 256kb of L2 cache. Doesn't look like it's quite as fast as the 1ghz PIII, but still looks like a terrific chip. It's based on the Tualatin core. And apparently, thanks to once again a new chip manufacturing process, it's easily overclockable to 1.5ghz.
But no thanks to the new manufacturing process, you'll have to buy a motherboard that specificially supports Tualatin (which looks like it will have a very short shelf life).
Computers just blow me away. Seems like just yesterday I was chomping at the bit to get the new Celeron 466mhz, for something like $170. And what an updrade that was, from my old Pentium I/200mhz.