The web is changing. For a couple of years now the shift in the way people use and develop content on the web has been called a number of things, most commonly web 2.0.
One of the shifts that has occurred is something that the average user (and often client) may not be aware.
Along with this shift in complexity of code needed to run these web sites comes all the other challenges introduced by complex software systems; collaboration, process, and tools.
The interesting thing to me is that the collaboration skills and process skills should (and seem to) be transferable. Whereas the tools and technologies may still be immature.
If you are looking to get a new Web 2.0 site put together then you should be thinking about all the things you would consider if you were asking for a client application to be built. There will always be a functions vs. time and resources trade off to consider as well as making sure you break the project into small manageable portions.
This new world on the web is not a simple case of slapping some images and text in markup but rather a full development process.
Many of the traditional 'online agencies' that have built a business model around slapping images and text into some markup and selling that to clients.
As more functional demands are made from web sites I expect these agencies will have to change their process or go out of business.
The web is finally starting to grow up into a real platform and running on top of this platform are applications build by software development teams (which include designers).