This is certainly welcome news for the AJAX world (which owes its existence to Microsoft, who introduced the XMLHttpRequest object in Internet Explorer 5.0, back in 1999). Libraries like Dojo and jQuery represent a kind of programmatic Switzerland for the AJAX world, shielding programmers from disparate browser behaviors that otherwise tend to make AJAX programming problematic for the non-ninja. Microsoft could easily break Dojo, jQuery, and the rest of AJAX-kind if it were to make (further) unwelcome changes in the way IE supports things like DOM and XMLHttpRequest. The fact that Redmond is on board with jQuery means the status quo will probably be preserved. Or at least, that's the signal it sends.
What will be interesting to see is whether Microsoft begins bundling (and providing 24/7 support for) other open-source products... and keeps its promise not to fork people's code. Is jQuery the beginning of a trend? Or the beginning of the end? We should know more in a few months.