I know for sure we've mentioned this on the podcast (in the ever-more-popular Turpster's Tips feature), but I'm not sure we've ever explicitly mentioned it on the site before, and this post from WoW Ladies is a great reminder to do so: next time you're checking out your character screen, try holding Alt as you mouse over the various loots you're wearing. You'll get a little popup with all of the gear in your bags that corresponds to that slot -- a (mostly) undocumented feature that showed up in patch 3.1 along with all of the other UI improvements.
In fact, that's probably why this is just sneaking around now -- there were a lot of nice tweaks back then, and this one slipped through the cracks. Of course, now that the official Equipment Manager is in the game (you have clicked that checkbox in the Interface options to enable it, right?), you might not need to worry about switching your gear manually anyway. But just in case you haven't seen the Alt function on your character screen yet, there you go.