AOP is an even more extreme version of XP, in many ways. Aspects deal primarily with system-wide functions such as logging and persistence, and allow you to write your core business logic without worrying about these things. Given XP's mantra of "don't worry about it until you need it", AOP is a perfect fit. You can add logging, for example, to your entire application after the fact, completely non-invasively. You need an AOP aware language such as AspectJ, but it's got some really interesting ramifications.
On the other hand, my guess is that AOP doesn't fit in particularly with test-driven software, since an aspected class doesn't know it's aspected, so I can't imagine how you would know to
unit test the externally aspected characteristics of a class.
Joe