I dunno, probably not real soon. Some webcomics people have done pretty well selling T-shirts, but generally the T-shirts have little or nothing to do with their comic strips; they're usually just funny/hip sayings or plays on words, or parodies of pop culture. I have no interest in doing that. I like to think that I would never produce something that I myself wouldn't want to buy. (I have nothing against those kinds of T-shirts, by the way, I just wouldn't be interested in owning one, hence I have no interest in producing one either).
If the strip was really popular, I could see making shirts, I'd just want them to be about the comic strip, promoting the comic strip in some way. I don't think at this point I'd be able to sell enough of them to make it worth my while. If Keenspot approached me and said, "the world desperately needs a Miriam T-shirt" I'd say go ahead and make one, but I'm not holding my breath waiting for that to happen.