Here's a demo video:
Tweets are "pushed" to an Arduino via USB cable from a computer running a Python script.
You can easily modify the script to display anything you like! (stock ticker, news headlines, free disk space, etc.)
This project is built upon my "Arduino Laser Show with Full XY Control" Instructable:
It's fairly easy to put together (figure an afternoon) - and is built with parts you can most likely get locally (it uses audio speakers to aim the laser).
You'll also need a computer connected to the internet (this project has been tested on both Windows 7 and Mac OS/X Lion - Linux should also work).
Once you've built the laser show itself - making it display tweets is just a matter of some software. Figure an hour or so to get it working.
This entire project can be built for around $50 (not including the cost of an Arduino and computer).
Step 1: Go Build The Laser Show
Step 31 details how to upgrade to a green laser.
Then come back here. No need to rush - I'll wait.