If your thinking about using DNN (thats Dot Net Nuke) I thought you might want to get a few pointers as to why we think it's worth considering...

1) huge community that seem keen to offer help
2) a really good basic admin management for registration and privilegies
3) developed in microsoft.net so a good pool of talent out there to extend it
4) lots of extra modules (though some are poor quality so try before you buy)
5) skinning options include lots of third party skins
6) a really generous open source license

We have been developing in Dot Net Nuke for many years now - its made a huge difference to the speed at which we can develop web based systems. It's a big system to get to grips with but worth the effort.