Shaun Walker, Creator & Maintainer of the open-source DotNetNuke project, has posted a "Feature Matrix Showdown" on the DNN site comparing various features of the following solutions:
- DotNetNuke 4.x
- ASP.NET 2.0 (out of the box)
- SharePoint Portal Server 2003 (and Windows Sharepoint Services 2.0)
- Microsoft Office Sharepoint Server 2007 BETA (and Windows Sharepoint Services 3.0)

This is an interesting comparison, and a good starting point for anyone in the market for a portal solution.