Hey Jason,

I'd recommend using what you're comfortable with. Most modules/skins are not XHTML compliant, but are becoming increasinly compliant. I would tend to recommend using a less restrictive doctype to accomodate a wider range of content. Also - your skin generally defines that doc type to use, in DotNetNuke 5 you can define a default doc type to use if one is not supplied.

I guess the point being - if you really value XHTML compliance, you can acheive it, but its quite a bit of work - especially if you don't control all of the modules/skins/content. If you do control everything (or are willing to "put the time in" with 3rd party  modules) you can definitely use XHTML doctype.

I'm sure someone else can pick this up and run with it too - that's just my rudimentary knowledge of the situation :)

Take care,

Ian