Typically in a release cycle, where you go from CTP to Beta, to RTM, the feature set is fixed by one of the early beta releases. WIth IE9, there have been a lot of releases and with each release there has been new features.
Was this something that was planned or did your team run across low hanging fruit and just decided to grab it as you went along?
...Here's another one...
The IE team has boosted performance and submitted many conformance tests for browsers implementing features of CSS, HTML, or x-Script, but from what I can tell, has not released any libraries for using these features in a coherent higher level way. Other MS teams have summited code to popular libraries like jQuery. Does the IE team plan on releasing a cross browser compatible library that really sings under IE?