I found this shirt while surfing the Canonical Store
The shirt is decorated with art based from the latest Ubuntu Hardy Heron wallpaper making it nice to be worn on LUG or Ubuntu meetups.
The downside is probably that once Intrepid Ibex comes out, the shirt would be the thing of yesterday, but rest assured as you can still wear it for other activities due to the generic nature of its design.
As for me, I would consider one if the shirt is available on the local market