I don’t know how long this has existed (or what to call it), but in 10.9.1, just as one is able to quickly retype previous commands in Terminal by pressing the up arrow key, it is possible to retrieve a copy of entire, previously typed messages in Messages.app with a similar key command.

Just press Option+Up arrow while the cursor is in the message-typing box. How many previous entries it will resurrect appears to depend on how much of the message history is currently in memory. If you scroll further up to load more of the message history, those additional messages become available to the key command.

[crarko adds: I believe we called it the command buffer in Terminal.]

Source: Mac OSX Hints