Cosmetically, the new PIIIs look a little different than previous PIIIs because they're sporting an Integrated Heat Spreader (IHS) a la the Pentium 4. For the terminology-weary among us, IHS is just a fancy name for "metal cap." The IHS covers the chip and, presumably, spreads heat.
There is a hole in the IHS, you may have noticed. An Intel rep once told me that hole is there to allow gas to escape as the newly-made chip cures. Either that's true, or he's been chuckling at my sadly mistaken notion of a farting chip. Whatever the case, the hole doesn't present any problems; if you get a little thermal paste down there, it's no big deal.
The new PIIIs fit into good ol' Socket 370 sockets, like so many PIIIs before them. However, because of clocking, voltage, and signal level differences, Intel doesn't recommend using these chips in older motherboards. Our test system included a B-step revision of Intel's 815 chipset, which is able to support the new PIIIs properly. Intel bills the motherboard, the catchily-named D815EEA2, as a universal Socket 370 platform, because it's able to accept anything from the oldest socketed Celerons to the newest Tualatin PIIIs.