A more recent version of scripts may be available at scripts.irssi.org.
Version: 1.0
Download
View source
signature
Keeps a prompt (the part where you type text) per window.
So if you're in window 1, start typing "hello", switch to window 2, your prompt is empty again, and when you switch back to window 1, you can continue where you left off there.
Note that keeping the history (what you get when you press the up arrow) is already built into irssi. You can enable that with
/set window_history on