I don't think pdadb.net has any insider info. It looks like the dates there are not very reliable (for the HD7S it lists a release date of "April, 2011").
Plus, I don't think it's likely that MS will release the update on the same day of their event. As far as I know they haven't even released Mango to developers yet.
I gotta agree with Thed. May for Mango would be well ahead of schedule. Isn't Mango planned for Q4 with rumors of it slipping into next year?
Regardless, I can't wait to see the IE9 demos.