Conditional comments do exactly that. There is an example in the jQuery blog post.

What are you specifically worried about? Dell is still using jQuery 1.2 on their site from 2007, and it seems to work. So in 2014, something will break in the (supported) 1.9 version that the jQuery team refuses to fix, but you can't upgrade to 2.1? Can you say what that something might be?

True, Walmart.com will have IE 6/7/8 visitors for years to come, but there are plenty of places where someone might be using jQuery but not need IE 6/7/8 support. It could be a cutting-edge public web site, a desktop app using an embedded HTML rendering engine, or a mobile app where oldIE doesn't matter. You can either use jQuery 1.9 for now, or use graceful degradation to give those people a really basic experience and a nudge to upgrade.

As far as bugs and quirks go, a lot of the ones in IE6 and IE7 are also present in IE8. In specific, things like eating HTML5 tags, the lack of true opacity support and the proprietary IE event model that used attachEvent instead of addEventListener. There's quite a bit of code in jQuery to deal with IE event issues, for example the lack of a bubbling change event. IE8 managed to plug some memory leaks and wasn't as bizarre with the "attroperties" issue as IE7 was, but they still have a lot of sins in common.