The new UI has several compatibility issues with IE 7.

+ The border images are screwed up and displayed incorrectly.
+ The message area scrolls off the screen (on resolution 1280x1024) (ie, when entering new messages like this, the blank message field goes off screen)
+ In forums the images for the moderator and users on are cut off by the messages window... in other words, the modules dont scale properly down so information displays over each other.
+ Images are not properly resized. If you click on someones photo album, all the images are full size regardless of the resizing scripts. Meanwhile the normal page text and formating remains, so images are covered by the text. On dark images its hard to locate the options to view full size, flag, comment a picture because of this.

This is on IE 7 beta 2, released yesterday. Granted its still a beta, its considered a release canidate, so might be worth looking at getting stuff to work with it before it goes mainstream in a couple of months.