Backyard ballistics are just too much fun. Don't miss this awesome page over at Format C: describing the design and fabrication of a fully functional replica of a medieval siege engine. The designer was inspired by this picture from Wikipedia:
And created the replica from the LEGO Technique sets (The Toy of the Century).
Here's a video of the weapon in action:
For those of you interested in more modern LEGO weaponry, check out this project that created a LEGO machine gun that shoots 96 bricks per minute from a 16 round magazine.
I'm not sure where to find more details other than an attribution to "Billy Glenn." But note, you can make ANYTHING with LEGOs.
Here are some other siege weapon links for the hobbyists among you:
Ripcord's Trebuchet Page
Nice Google Video on Large scale Trebuchet models