What is it? No more design up front? Blaspheme!
Most Architects of the day would scoff at the thought, the idea of an evolving architecture seems counter-intuitive, if I’m building a house I cant just keep changing the foundation…. Can I? Well I can, but it will have an effect on the house. Just as in Agile development, all change have an effect - It’s about weighing up the effects with the pay offs.
Picking a proven, lightweight framework to get the minimum requirement's done. Use this first step to get the conversation and tight feedback looping going from as early as possible. Feedback loops with stakeholders are vital, if you can get your lightweight framework done in a sprint (from project inception) means that you’re already getting feedback on what you system can do – and what your stakeholder wants it to do.
Your best base architecture is any architecture follow at least all SOLID principles and patterns and practices. All problems resolve to common patterns (some unique situations – but there are exceptions to every rule)
I found this talk helpful in articulating the process of Agile Architecture, in particular the Command and Query pattern / ideology.
http://pluralsight-free.s3.amazonaws.com/pluralcast/pc_045_mario_agile_arch.mp3