Support staff (wisely) do not waste time trying to deal with issues involving beta versions of browsers. Which is why I classified this topic as "not a support question."
But, I think it might be useful to have a thread mentioning problems with Internet Explorer 9 Beta.
In my case I cannot log in to any of my WordPress blogs/sites. But I can do it in all other sites I have tried so far: Facebook, Google, YouTube, my (non-Gmail) email account, and so on.
Nor am I mis-entering the passwords, as by having the Caps Lock key on.
It also has nothing to do with whether you have HTTPS enable for logins via the Users--> Personal Settings--> Browser Connections option.
It is not important to me, because I use Firefox for all work on the several sites for which I'm responsible. But whenever I have made an important change on a site I do check it out on IE, Chrome, and Safari as well as Firefox.
Has anyone else noticed this problem?