I downloaded this Pomodoro Timer from the Apple website.
It has support for AppleScript events, so I created a script that automatically sets my Adium status to away (with an auto-reply) while I am working on a Pomodoro and automatically sets it to Available when I have finished.

After much messing around, trying to work out just how the AppleScript pseudo-natural-language works, I ended up with this to set the away status:

-- This goes on one line in Pomodoro Setup -> AppleScript -> Start
tell application "Adium" 
  to set the status of every account whose status type is available 
  to the first status whose title is "Pomodoro In Progress"

and this to set it back to available:

-- This goes on one line in Pomodoro Setup -> AppleScript -> Reset and End
tell application "Adium" 
  to set the status of every account whose status type is away 
  to the first status whose title is "Available"