First, thank you all for your support in answering my question.

Second, let me answer my own question based on much additional research.  DNN is absolutely a serious, powerful and widely supported platform.  Take a look at some of the sites referenced in this thread as evidence.  Also, go out and buy the book Professional DotNetNuke 4: Open Source Web Application Framework for ASP.NET 2.0 (Programmer to Programmer).  When you get it, read the history section at the beginning of the book written by Shaun Walker himself.  After reading this, I have much respect for both Shaun and the application itself.  This is not some half-baked idea cobbled into a makeshift application.  This is the real-deal. 

I'm sold and I'm serious about DNN.

Thanks to you Shaun and everyone involved for bringing this to where it is today.