Since I joind the HTML WG in March, all messages from a microsoft.com e-mail address are Chris Wilson's, except for one message from Paul Nelson (@winse.microsoft.com, whatever that means). Chris was quite responsive regarding Microsoft's position on whether HTML is/should be a versioned language. Apart from this, Microsoft is invisble.
IE6 had this too. That's why you added a workaround (_width: 1%). But IE7 won't recognize _width (which is correct). #navigation #contenttypes li a { float: left; } would do the trick. But I suggest you leave it as it is and ask Microsoft to fix this ancient CSS1 bug.