The cycle here is pretty long:
jQuery 2.0 will be released in "Early 2013". For some time (probably at least 6 months if not longer), jQuery 1.9 and 2.x will be perfectly API compatible. That means that the conditional comment approach will work with very little downside.
That gets us to summer or Fall 2013. At this point, there may be a release of jQuery 2.1. According to the blog post, it will probably contain few new features, and some speed improvements. It will almost certainly not break backwards API compatibility. At this point, you can upgrade to jQuery 2.1 for the speed improvements and simply not use the new features. The conditional comment approach will still work.
That gets us to Spring 2014. At this point, Microsoft will EOL Windows XP. It's frankly hard for me to imagine more than a tiny number of Windows XP users using IE8 at this point. There was a lot of similar-sounding concern about IE6, which has mercifully vanished. IE8 was at 30% last November; it is now down to 15%.
Even at this point, a jQuery 2.2 will likely not break API backwards compatibility. If IE8 clingers are more stubborn than we expect, we can always modify the strategy to be more aggressive about strict compatibility between the branches.
Unfortunately, many of us are still fighting the last war, where the cutting edge (IE6->IE7) was plodding and the slow upgrading tail was huge. It's time that we acknowledged that the ecosystem has changed: the slow upgrading tail grows smaller all the time, as the cutting edge absorbs more of the mindless browser population. At the same time, the pace of the cutting edge grows faster and faster. We aren't quite there yet, but we should prepare for a future that is not a simple clone of the 2004-2008 era of browser development.