The chip
Beyond the internal changes, the Athlon XP comes in a new package. It looks like so:

Top view of the Athlon XP 1800+

Bottom view: some new clutter underneath

The Athlon XP's new outfit is an organic pin grid array (OPGA) package. This package has a fiberglass substrate, like a printed circuit board—and quite similar to the packaging on Intel chips. AMD claims the new organic packaging is cheaper to produce and offers lower impedance than the ceramic packaging they've used in the past.

It's also quite brown.

I'm hopeful the new packaging will flex and bend ever so slightly, preventing the disasters with cracked or chipped cores we've seen with Athlons in the past. However, the Athlon XP's core is just as exposed and vulnerable as ever. I had hoped to see a metal cap ("Integrated Heat Spreader") a la the Pentium 4, but no such luck.

From left to right: Athlon "Thunderbird", Athlon MP, and Athlon XP