I hear this question still today a lot from several customers, when looking at using Microsoft technologies like SQL Server, IIS or Windows Server for building their line of business application and some applications that are really mission critical. Let's look at question from a few angles.
It would seem like every application being developed today is mission critical according to the ones who are developing the application. When teams talk about mission critical they often times suggest the application is also big and complex. Mission critical does not equal complex or hard. Just because the application is important (or mission critical), this does mean that the applications workload is large, the business logic comples, or the application is CPU or IO intensive.
Most of us who have evaluated or worked with Microsoft server technologies are used to working with 32 bit code, where even then most of the apps you are building scale just fine. Keep in mind today's world and the next generation is all about multi-core, and 64 bit code and the product teams are fully exploiting this capability in products like SQL Server, Windows Server, Exchange and other related server products. This means your code gains a tremendous boost in performance for both concurrent users, memory and cpu intensive workloads.
In general unless you are developing an application that is designed to handle hundreds of thousands of users and some serious memory and/or cpu intensive scenarios, then this question is a waste of your time. If your organization allows you to use Microsoft server technologies and it makes sense for your project from a cost, and supportability point of view, focus on more important things like actual project requirements, then worrying about if 64 bit SQL Server in a dual core multi CPU server is scalable (trust me you are worried about the wrong thing in most cases). The question is more about how you are going architect your deployment versus can the server technology handle your workload.
Someone may attempt to make the following argument; I am creating a public facing web site with millions of users can Windows Server technologies scale to meet my public facing web site requirements. One response for that question (microsoft.com; msdn.com; live.com; msn.com; msnbc.com; to name a few). If you are building a bigger more complex site than anyone of these then maybe the question is worth your time, otherwise let's work together to figure out the approriate deployment of Microsoft server technologies.