I was looking for some Haiku stuff on http://www.cafepress.com/haiku_os and to be honest the stuff looks a bit outdated and… ‘elderly’(?). T’s look a bit like something someone in a retirement home might wear… Maybe some folks from the forums’ ‘Creative Design’ section could maybe ‘up’ the stuff a little? I could, but I’m not any good… 
I think they have gone for the most non-offensive Designs possible. This can kind of make it a bit bland and as you put it “something someone in a rest-home might wear”.
And I think you do have to have images somewhat simple for Cafépress.
Yes, Cafepress is a little bit limited in what they support. I believe there’s support for full front/back imagery on the T-shirt at least, but it will increase the costs a bit to do it.
Note that the imagery on the “Golf Shirt” is a little less bland (was designed for our LinuxWorld appearance). It originally had some back-side graphics as well, but we removed it for the store due to the cost, and general opinion that golf shirts should be more work-appropriate ;). I think we would be willing to shift the rest of the shirts over to this design as well, and I’ll discuss it internally a bit.