tell application “Twitter”
set mention to text of first status of mentions timeline of current account
set sender to name of author of first status of mentions timeline of current account
end tell

With regard to triggering it, options I can think of…

1) Configure your script to run on a schedule, i.e. once every few minutes, to check for mentions.

2) See if you can find a Twitter client that has either (a) the ability to execute a script on a mention, or (b) the ability to attach a script to an event using an event handler. Unfortunately, I can’t point you in a specific direction here, as I haven’t investigated these capabilities in the various Twitter apps.

3) See if you can tie into Growl and get a script to run when the Twitter app displays a Growl notification. I don’t know whether this is actually possible, just speculating.

Another thought I had was to try and see if there’s a way to access an RSS feed of your mentions. If so, then you could use something like Reeder or Google Reader to be notified of mentions.

Hope this helps. Good luck!

-Ben