Two questions continually haunt with with Agile practices:
1.) How do you handle fixed bid/fixed scope projects without affecting quality?
2.) Where and when is the architecture defined?
In this article Kent Beck provides his perspective. Well worth the read.
"My experience with architecture shapes the architecture strategy in XP, I repeatedly ran into architectures that were developed up-front, without feedback, and by groups which didn't have to live with the day-by-day consequences of their decisions. What I observed about successful architectures was that they were grown little-by-little." - Kent Beck