When it comes to using the 3 Laws of Platform Adoption as a metric I think DotNetNuke stacks up pretty well for your small to medium business. Lets take a look at the condensed version of the 3 Laws.
1. Developers adopt a platform when it offers differentiation from competitors
2. Developers adopt a platform when it reduces the cost of software development
3. Developers adopt a platform when it provides reach and/or better distribution
So, starting with the obvious number 1, what differentiates DotNetNuke from its competitors? For me, DotNetNuke is different than its competitors in that it is designed to run atop your existing Microsoft software without adding any licensing fees. Small business clients running *nix servers are few and far between, so the ability to leverage your existing technology resource investments gives people a warm and fuzzy feeling. As far as Sharepoint as a competitor is concerned, it just doesn't make sense for small businesses because of the licensing fees. I know that Sharepoint Services 3.0 is free to download for Windows Server 2003, but most of the cool features are only available by ponying up extra for MOSS.
In the world of providing custom solutions for small businesses, Cost = Time = Money. This is why DotNetNuke also has a good footing for number 2. From a developers standpoint DotNetNuke runs on very familiar, some would say it's even cheap (as in inexpensive), technology (Visual Studio Express, IIS, Windows Server, Vista, even XP), which means less setup, implementation, deployment and maintenance time. Developers can use the .Net language they are most comfortable in (C# all the way!) and utilize the functionality that DotNetNuke comes with right out of the box. Functionality like user profiles, role based security, multiple authentication providers, secure file access, and rich content editing.
DotNetNuke also has a strong and innovative community of users. The DotNetNuke Marketplace allows developers a place to distribute their products and services to a wide range of customers (current registered user base on DotNetNuke.com. The DotNetNuke core team has made it easy for users of DotNetNuke to find products in this marketplace by way of a link to the "Solutions Explorer" that is standard in all DotNetNuke installations. DotNetNuke acts like an open source community with a business mind. Along with the Marketplace, the introduction of Service Level Agreements that are competitively priced give other businesses the opportunity to make profits on helping users with their DotNetNuke problems.