We're overdue an update to the list of which browsers our website officially supports, so here's a quick run down:
IE represents about 50% of all visits to our site (that's visitors from both on- and off-campus). Of those, 75% are IE8, this is followed by IE7 and IE9 in that order. Of our IE6 visits, there are twice as many users in the UK as there are in China.
A quick comparison on caniuse.com shows that by switching to HTML5 we aren't going to use any features which could be used by our biggest audience by percentage, but if we're smart, we can start to target standalone apps which don't necessarily use our main (XHTML 1.0 Transitional) doctype, or have a high percentage of non-IE users, and introduce new features to them using tools like Modernizr.
IE10 Platform Preview 2 has just been released and this does contain some basic HTML5 functionality such as support for some of the new form elements so it's definitely worth getting ready for the future. We'll probably create test areas of our site, not available to the public, which use the new HTML5 features so that when the new browsers come out, we can easily see whether they're yet good enough for our purposes.