I don’t have a method of installing old browsers, so I’m relying on an online service to test (take it for what it’s worth). Looks like IE does say that an upgrade is neccessary, but Chrome and Firefox don’t:
Chrome 10
Firefox 10
IE 9
As for your comment that XP won’t work: are you saying that if I installed Chrome or Firefox on XP you would detect that somehow and not let Discourse sites load?