i've wrote a nice perl script, which compares USE flags used during compilation of each package with currently set. it ought to be for my own at first, but i've really liked it, so i done some fancy stuff like color output, comments in code and so on. test it if you want and if you'll like it, feel free to put it into portage as separate program or as a part of some package (gentoolkit maybe?)
Created attachment 34878 [details] useupdate script
Already in portage, emerge's --newuse option. ;>