Like many IRC clients and developer utilities, iChat 6.0 in Lion makes it easy to scroll back through the messages you have sent to a buddy and re-send them. With your input focus in the text box, hold the Option key while pressing the up arrow to scroll back chronologically through each message you’ve sent. The messages populates the text box as if you had just finished typing it, allowing you to strike Return to immediately send it again or edit it before sending.
This can be quite useful if have to repeat something like a link or a complex set of instructions, especially if your friend is mobile, closed their chat window, or otherwise cannot access that portion of the chat history.
Unfortunately, even if you enable iChat to save your chat history (it’s still off by default in Lion), iChat does not provide access to your input history between login sessions or even chat sessions; close the chat window, and the Option-up command silently mocks your attempt at reliving chat history. Your input history is also not available across multiple buddies—what you say to one buddy is not Option-up-able in another chat that is happening simultaneously.