Autodesk is running a promotion to raise interest in their new web-based 3D modeling tool, 123D, which we've written about before.
You don't get a t-shirt for nothing, though. You have to make an image of your original 3D Jack-o-lantern design and share it within their online gallery. This could be easier than you think if you make use of their starter pumpkin models and instructional video. Then they'll send you a t-shirt.
The program ends (presumably) at the end of this month.