Scott Guthrie's team is one of the most forward-looking groups at Microsoft, and I think the developer division makes the best Microsoft products to begin with: Visual Studio and a few essential plugins is a much better IDE than Windows is a consumer operating system.
At a high level, jQuery has two main things going for it:
1) It has awesome functionality.
2) It's indistinguishably close to being browser-independent
It looks like MS has adopted jQuery for (1), and their developers will get (2) for free.
Of course, (2) may also have figured into MS's (apparent) decision to adopt. If that's the case, more power to 'em. Nothing would please me more than seeing (one division of) MS get a clue.
Congratulations to the whole jQuery team. They are constantly amassing success stories and they could not be any more deserving. They put out a great library and foster one of the nicest developer communities on the web.
I think Microsoft is offsetting the pain developers face in trying to make web pages work in IE/JScript by promoting jQuery. Without a library like jQuery/Prototype its really difficult to get IE to do what you want. Good news.
