...Okay. It's not an easy job to align products and release cycles. Microsoft was already late on WP7. They probably missed a cycle and Android kicked in. Microsoft doesn't want to be late anymore. Meaning, they probably couldn't wait for IE9 for mobile to be released (which wouldn't be a HUGE job if you have a working, stable, and RC version of IE9).
In conclusion, Microsoft is being smart that they release the phone and let people dive/absorb/digest earlier instead of waiting for full featured phone OS with a HTML5 supported browser, which is still in beta for desktops. However, this just means..the next release of WP7 will be very interesting. Remember, Android and iPhone OS (which wasn't iOS at the time) weren't closed to feature complete with their first release. I'm an Android user, and I can remember when I first bought my Droid, it went through so many updates software updates. Companies just have to cut things if one or two things will block the entire release.
http://moishelettvin.blogspot.com/2006/11/windows-shutdown-c...
Sounds like it takes years to get the legs moving together, which means it's too late for IE9 on the phone for a while...