Let's not get ahead of ourselves. The iMessage client can't keep messages in order or search them, let alone do anything complicated like that.
Message search is also built into the iOS spotlight screen (left of first home screen).
ED: I tried searching for a word in Messages at five minutes ago, and the results still haven't appeared. 100% CPU usage.
ED2: 10 minutes now, 2.24GB of memory use.
ED3: Results showed, then client crashed at 13 minutes.
Sidenote: apple has the ability to mine so many conversations for data. It's more intimate than twitter and I'd imagine more iMessages are sent each day than tweets. If something happens anywhere in the world Apple would theoretically be the first to know. Kinda nuts.