Its the first time MS is shipping an open source component they didn’t write with one of their products. This doesn’t seem like a big deal, but its almost nuclear in its implications.
Think about it… They are saying “we support this.” In an OSS product, MS has no control over the code. So, they are putting their livelihood (in some small way) into the hands of people who don’t work for MS.
I think jQuery’s popularity, the fact that it’s not mission critical code, and that the codebase is so small all came together to make for favorable circumstances for MS to dip their toe in the water.