Actually this is a feature I'm very interested in iOS 7. Currently, iOS 6 lets you search but only shows you results for recent messages, which is a hassle when you're trying to find something your friend sent you a year ago.
Also, the "load earlier messages" button only loads a small number of messages. With some friends I have 5 year's worth of texts (since my 2008 3G) and digging for an old text via the "load earlier messages" button is an exercise in patience that I always fail.
My question: is this fixed? Is that what your thread is referring to?