When your doing the testing of v4, please can you test that it works on a few mobile phones, especially windows smartphones.  (i do a lot of my browsing on my htc s620 so it would be useful, at the moment the site is unusable on it.  Something to bear in mind with the headers and layouts)  Also please consider any videos having a link for downloading to mobile windows media player. (I know there is a tradeoff with download size and image resolution... maybe have a mobile content button for it or something.)

I think in the long term you will find this beneficial as the current trends in web browsing seem to be done a lot more on mobiles than they used to be.  If you need any advice on designing for mobility then I'm sure the mobile patterns and practices team will be happy to help and advise.

Many thanks
Sarah