I am looking more heavily HTML 5 now, but I keep coming back to one issue and that's the practicality of using it without shooting myself in the foot.
Most of the site I work on are for SMEs that target customer bases that typically use IE7 or 8 and a surprising number don't enable JavaScript- something stupid like 10 - 15%.
This means I cannot rely on visitors having up to date browsers and I can't rely on the JavaScript fixes (such as modernizr) to compensate. I've seen some very powerful arguments for how HTML5 can really speed up Dev time and makes things so much easier, but how do you maintain graceful degradation when implementing?
the extent of HTML5 implementation you're looking at is the key to a solution. let me illustrate this with the following:
take for instance now the HTML5's video attribute - it is a fairly common practice to put in place a flash fallback for incompatible legacy browsers so that regardless of the situation, everyone gets to view the video.
question here is: "so what do I use as the markup? HTML 4.01, XHTML 1.0 or HTML5?". the answer, naturally, would be to use the HTML5 markup because the video tag is in fact a HTML5 one. in part, the answer is not wrong, but it is also not completely correct; at this point of time when browser support for HTML5 is not full and the markup has yet to hit the recommended stage it will be unwise to mark it up at HTML5 because legacy browsers will just give you a blank look. Instead, if there is to be any graceful degradation of sorts, then your HTML document should be marked up as XHTML 1.0 or HTML 4.01 and then included with the desired tag, which in this case for an example is the video tag. Legacy browsers will render our DIVs and CSS styles correctly but will not recognize the video tag and will instead go to the Flash fall back video.
so i suppose that once you have a more targeted idea of the extent you'd like to use HTML5 within the constraints of legacy browsers and the lack of JavaScript support, you should be more than able to figure a way out
also, though HTML5's support an compatibility is not yet fully realized, it doesn't mean we should avoid it like a plague. give it a shot and fall back on some common methods like the example illustrated with the video tag (read diveintohtml5. it's a fantastic resource!).
just my two cents' worth. hope it's helpful some how.
(heres an example of a site that I designed and co-developed with an agency - http://www.blueprint.sg. it's marked up in XHTML 1.0 but uses the tag for a full screen background video. falls back to a backgroun image for browsers unable to recognize the tag. PS: apologies in advance for the not so good design and the annoying playing of the video which is currently set at "autoplay" so it'll play and load at the same time.)