Please have a look at the following article by Martin Fowler : Who Needs an Architect?. It provides an excellent view of what an architect is and what architecture REALLY is, i.e. somebody who makes a bunch of design models in UML is NOT an architect.
In RUP you look at architecture very early. Some of the most basic business requirements go right to architecture ... "Users include sales agents on the road in 50 states" tells you something very important day one. Architectural proofs can begin as soon as you think you know something like that. See Scott Ambler's modified lump chart or whale chart . That early "implementation" activity is architecture spikes. XP is into early architectural spikes, too, but it also talks a lot about deferring architecture decisions and letting it emerge. (No conflict - defer the decision, then spike the decision to make sure it works.) That's mostly a great idea (note: I'm agreeing here!) but XP is about generating code and doesn't often mention dependencies on other people. If it's going to take the network engineers months to get an infrastructure set up, you better think about it early!
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi