The ideas I've heard, and what has been built into Powerpivot with Sharepoint 2010 integration is a way to watch and see which applications are heavily used and they can be rewritten as a true SSAS application or enhanced.

Great points raised above, and I don't think this eliminates DWs as there is still a need for those large, clean sets of data. Without them, it is a tool to allow someone to make mistakes much, much quicker and with more confidence.