I am having the same issues with typing replies as others. If I back space, the computer seems to hang.
But, in general, the whole Channel9 site seems to be bad for IE. If I scroll through the main page or the forums, it is very choppy and lags happen throughout. My processor spikes to 100% as I type replies and scroll through pages. This experience is not happening on other broswers I have used on this site.
The lagging/processor spiking also occurs when I click on the Favorites Center icon and then click an RSS feed to read.
I am having no problems with Live.com/mail beta as others have mentioned.
EDIT: I have also noticed that when I open a new tab, the GoogleToolbar, I have the newest version (V4), does not show up until I actually go to a website with that new tab. Is this behavior by design?