Though the 4.4 documentation is lacking Cathal Connolly's major strides towards compliance in the core are begining to show in the releases, this new feature being one of them.

The Gemini entry for this enhancement is here (implemtation notes are in the comment of the entry):
http://support.dotnetnuke.com/issue/ViewIssue.aspx?id=3623

Check out his other UI related entries in 4.4!
http://support.dotnetnuke.com/project/ChangeLog.aspx?PROJID=2

Thanks very much Cathal!

Previously we had to define the doctype in default.aspx, unfortunately altering the core. I think this is exactly the place to implement doctypes, with the skins.

I'm curious though, since I develop skins straight into ascx files and not into html & xml, do I create the previosly un-needed xml file and then re-parse the ascx using the DNN gui or can I just add the tag to my ascx skins now and DNN will pick them up?

I blogged the details @ http://www.dotnetnuke.com/Community/BlogsDotNetNuke/tabid/825/EntryID/1226/Default.aspx , sorry for the delay (xmas turkey induced coma :) ). I'll update the skinning documentation when I get a chance. Ill also blog a little about the reasons for the current implementation and some other bits and pieces tommorow (i've received half a dozen emails  already so I'll aggregate my answers into a blog post)

I originally found a note on the Microsoft site advising against using an explicit DOCTYPE declarations in the ascx file itself (as an ascx is a page fragment it can cause a failure to load). As John notes, as doctype is now controlled by a server tag, you can of course update the reference via the ascx i.e. skindoctype.text="....", which is safer as the page event model will handle any multiple declarations.

Cathal