Recent events have prompted me to make an entry about scalability. It can be summarized with the title of this post. Scalability is a function of design, NOT platform.
I won't bore you with statistical analysis of Java vs. .NET. A quick Google search will reveal all you ever wanted to know. I want to spend just a minute focusing on scalability. http://www.whatis.com defines scalability two ways and I want to focus on the first. Basically, scalability is the ability of a software system to provide roughly the same performance / throughput / etc. under ever-increasing load (to a point, of course). There are many factors that impact the scalability of a system such as data access techniques, caching techniques, security, distribution of business logic, separation of presentation logic, and external system integration.
Java and .NET are VERY similar (and I've done both) in terms of the framework / class libraries they offer. They each offer a range of options when approaching the aspects described above. There are case studies demonstrating that each technology's approach has merit and can be used efficiently and effectively. I can write scalable or not-so-scalable software using either technology. I'm sure there are plenty of others out there who can as well. I'd love to read your ideas on scalability and the Java vs. .NET debate!