Bewarned: this post has to do with a technical, hidden process in your WoW client, something involving how your computer's memory is used and how you might be able to make it a little faster. But if you're not interested in the technical side of things, don't worry: this post is extremely skippable. The upshot is that while you can download an addon right now to possibly make your memory usage a little faster, the same functionality is coming to the regular client in 2.3. So feel free to move on to the next post if that's all you wanted to know.
If you're still reading, you probably know by now that we're talking about garbage collection. Cladhaire has a well-written thread started in the UI and Macros forum that goes over exactly how garbage collection works in your computer's RAM-- after Burning Crusade, the WoW client uses a process called "incremental garbage collection," which dynamically deallocates memory as it falls out of use within the application. The problem, as I understand it, is that sometimes the memory won't actually fall out of use, which means the garbage collector won't kick off, which means garbage will sometimes sit in memory for a while waiting to be collected. For those times, there's GCTweak, an addon which you can install that will occasionally force the garbage collector to do its thing. If you have really low system memory, this might help, but only slightly.
At any rate, while this is interesting (probably moreso to those of you comp sci majors out there), it's really a nonissue-- Slouken confirms that exactly this functionality is being built into the standard client in 2.3. If you're really good at this technical stuff, GCTweak might let you tweak your system enough to pump out a few extra FPS on a lower end system. But if you don't know RAM from ROM, and can't be bothered with all this technical wackiness, just wait until 2.3, and you'll get this same functionality anyway.