Architects on an XP team look for and execute large-scale refactorings, write system-level tests that stress the architecture, and implement stories. Architects apply their expertise a little bit at a time throughout the project. They direct the architecture of the project throughout its evolution. The architecture for a little system should not be the same as for a big system. While the system is little the architect makes sure the system has just the right little architecture. As the system grows, the architect makes sure the architecture keeps pace.
Making big architectural changes in small, safe steps is one of the challenges for an XP team. The principle of the alignment of authority and responsibility suggests that it is a bad idea to give one person the power to make decisions that others have to follow without having to personally live with the consequences. Architects sign up for programming tasks just like any programmer. However, they are also on the lookout for big changes that have big payoffs.
Tests can communicate architectural intent. I talked with the architect of a major credit card processor who said that in such a high-capacity environment you don't want any architecture that might get in the way. To achieve this his team had a sophisticated stress testing environment. When they wanted to improve the architecture they would first improve the stress tests until the system broke. Then they would improve the architecture just enough to run the tests.
I suggested this strategy to an architect at another company. He complained of spending all of his time writing specifications and then explaining them to developers. He was frustrated that he didn't have time to code any more. I suggested he write a testing infrastructure and then use tests instead of specifications and explanations. If he saw a hole in a design he should write a failing test to point it out. I wasn't able to convince him to try the idea, but I still think it's valuable.
Another task for architects on an XP team is partitioning systems. Partitioning isn't an up-front, once-and-for-all task, though. Rather than divide and conquer, an XP team conquers and divides. First a small team writes a small system. Then they find the natural fracture lines and divide the system into relatively independent parts for expansion. The architects help choose the most appropriate fracture lines and then follow the system as a whole, keeping the big picture in mind as the groups focus on their smaller section.