I don't reckon the UI will change much from the Technical Preview. A few tweaks for things like Notification Centre. And something happening with Charms (if there weren't hardware buttons for them on quite a few devices, I could see them being dropped). Don't think we'll see anything related to phones.
MS are so focussed on support for platforms other than their own that it would be nice to see something like Office Touch to reverse the opinion that they don't value their own operating systems. I doubt it though.
Maybe some news on US-only features being made available to a few more territories.
And with any luck, they'll be some kind of huge, massive, extra-special apology for XBM on Windows Phone.