Funny it happened to me with Shorten script. There were two replies at first. Yada, yada, by the time I posted BrowserUk already posted a better answer--worse still similar to mine, which made me look like a plagiarizer :) (sometimes you read something and reply hours later).

Anyway. As for the "reply status" feature, if someone wants it to be more or less real time, at least two possible ways to do it.

One. Have a "hidden" frame on the page (i.e. a frame with zero height) which periodically requests the reply status and via javascript refreshes the reply status on the visible frame (on an input box for example).

But that requires a major rewrite of the PM script since it's not using frame.

Another more plugin approach could be using Java applet. Passive periodic querying the server. Or using callback having the server notifies the applet the reply status (just like IM). But callback may not be allowed on the client side (as a security measure).