Looking how to generally improve scaling and performance of ASP.Net applications shows how complex the problem is. Every problem will have a different solution as it needs to be broken down and analyzed to find bottlenecks and ways to solve them.
Some informational texts on the topic include:
One of the solutions can be to use the newly emerging distributed data layer. Similar to Scale-Out, NCache, and others, Microsoft has started the Velocity project.
To see MySpace architecture and how it solved the scalability issue, read here. It confirms that the right approach in building high-capacity scalable sites lies in correctly implementing the caching or distributed-storage technology.