Live tagging of chat conversations. Imagine chatting with someone, and hitting a button, and flagging that spot in your talk with a tag. "To Do", "Work", whatever. Let it be indexed automatically by your favorite metadata search engine. When you pull it up in Spotlight or Beagle, your conversation is indexed along with the rest and easily findable.
More than that: transmit the tag over XMPP while you're talking, and their client can suggest that tag, too.