Google confirmed the issue in the thread, they said they are in the final phases of testing Google Instant on IE9.
Jonathan E. from Google said:
We're is in the final stages of testing Instant on IE9 and hope to be able to enable it in the coming week or so. We'll keep you posted.
What is the hold up? Well, as Jonathan explained, "a new version of the rendering engine (Trident 5) that's part of IE9." Google said they "did come across some changes that we needed to make to support it."
So hopefully Google Instant will work in IE9 within a couple weeks.
Forum discussion at Google Web Search Help.