I'd rather see something more understated on the front: The LNS logo in the pocket area, and something like "Serving the Mac community since 1995." That's an impressive history.
Then on back some information about SD, including the bug. Part of this may be my own needs, because I can no longer wear t-shirts with a lot of ink coverage, especially on the chest.
I've only used cafepress.com, but have had good experiences. The shirts are not cheap, but you don't have to order in bulk, which is very nice. Individual users can order on their own. There is no extra charge, I believe, for full-color. So using the color bug on the back is feasible. The color bug will attract the attention, and then the marketing person in me says you should put some text with a brief description of SD6.
And, yes, definitely include the version number. It is our way of dating things. I think I only recently threw out my QuicKeys 2.0 shirt a couple of years ago.