I recently installed IE7. There was an issue with the localLinks, but that is solved. Today I ran into another problem. If I used the treePicker, it would give me a javascript error: this.xmlHttpObject.readyState is Null or not an object.
After a bit of testing I found that the problem was in IE7 itself. I solved it by disabling the 'Enable native XMLHTTP support'. After I did that, the treePicker started working just fine like it should. I found 1 person on the umbraco-forum that had experienced the same, but I bet there will be more that are using IE7.