I believe that design is still essential, regardless of the mechanism used to deliver the solution. There must be boundaries, especially in terms of defining the extent of a solution, and particularly around interfaces, integration, and simply avoiding straying into the space that other projects are attempting to cover - otherwise a strong PM / product manager could take the development in a direction that is completely wrong for the organisation or enterprise that is needing the solution.
A simple example may clarify what I mean. Consider a company that has a well designed, integrated, management reporting system based on a recognised tool, that works with industry standard databases. Within that company there is a small development that uses a different database, different data standards, and duplicates some of the functionality of other enterprise applications (including some management information). Not a problem... for the project team. But it is a problem for the organisation, as it now has multiple versions of the "truth", and no-one knows for sure which one is right, because they will never be perfectly aligned. Why has this happened? - because there was no-one validating the design or setting parameters for the developers to remain within.
So yes, software architecture is very relevant in Agile, especially in integrated, complex, enterprise-sized environments. But I would also argue for proper design in any development, as there is always merit in keeping at least one eye on the prize that you're shooting for, as well as creating the code for your next iteration.