Josh, even with the doctype by default nature DNN renders non compliant code. It tends to not close tags that it should, and at times renders a few controls that are not xhtml compliant. You have to tweak around with the files a bit, to get most of the errors out.
Jeff: I agree that CSS is a * self censorship * to get to look correct in all the browsers around. Always some little tweak here or there. However once you get the hang of making those tweaks it becomes easier to build CSS skins then table based ones. One huge benefit of CSS based skinning is the ability to make large scale changes with little changes to the base html. Granted you really need to know CSS and how to accomplish what you want.
Overall though its no different then when we first all started using tables, they were horrid to deal with didnt render the same in any browser (still don't today lol) the only difference is we have been using them for the last 10 some years so we know all the quirks to work out. Same thing will happen with CSS based layouts.
I personally like the ease of CSS based layouts, I can create some really impressive skins with about 10 lines of html and some CSS which would take me about 3 times the amount of code to do with tables.
I do however have a huge pillow I keep at my desk for when I want to slam my head down. No need to kill off brain cells.
Hopefully microsoft will wise up and make a compliance / standards based explorer before I pass away, but I am not holding my breath.