G'day,
The following can be tacked onto the end of the html keywords line in filetypes.xml for the HTML5 new elements to be recognised:
article aside audio canvas command details datalist figure figcaption footer header hgroup keygen mark meter nav output progress section time video wbr
What would be quite nice is to be able to have different modes for HTML/XHTML to allow for the defined keywords of each doctype to be recognised as appropriate, e.g. HTML <=4 and XHTML won't know what a canvas is so it should appear as an error in those doctypes, and HTML5 won't know what center is (which is just as well because it's misspelled in all but Wronglish :)
Admittedly that's a bit of a tall order, and I have no clue where to start -- but I might sit on it as a possible plugin someday. If somebody just happens to sort it out sooner, I reckon there'd be quite a few happy people about the place!
cheers, Ross