About a year ago, I attended Cloudforce in New York City. There were several interesting presentations, including the first peek at Salesforce CRM Content (which I believe is the foundation to Chatter). The most impressive presentation by far, was the one given by Salesforce.com's Chief Software Architect, Craig Weissman, titled True Multi-tenancy: A New Architecture.
This is a good video for any entrepreneur that is contemplating a new SaaS startup (the good stuff starts five minutes into the video). I don't think that Salesforce.com is doing everything right, but the bottom line is that their system was architected with scalability in mind from the very beginning, and it obviously paid off for them. It seems obvious that you would build a company thinking about that stuff, but it seems like many platforms out there that are not scalable, and once you get trapped with scalability issues, it is hard to fix the problem effectively.