Mark Lemley and Stacey Dogan have argued, quite persuasively on normative grounds, for a "trademark use" requirement before infringement can be found. Graeme Dinwoodie and Mark Janis have picked apart the statutory and historical support for such a requirement, but these days I'm leaning mostly towards Mark McKenna's primary argument (also made by Dinwoodie & Janis): as long as anything can serve a source-identifying function (remember Breyer's rejection of ontology in Qualitex), trademark use isn't a helpful limit.
Here's an interesting set of examples. Glarkware sells, among other things, merchandise associated with TelevisionWithoutPity, the TV recap/commentary/discussion site. This merchandise often refers to shows, though the references will be obscure to nonfans. So there are shirts for Smallville, Supernatural, Battlestar Galactica (on the first, the font is similar to the show font), Heroes (much more obscure in its reference than the T-shirt from a few weeks ago), The Daily Show, Lost (II), Jeopardy, Veronica Mars (the second particularly interesting because it initially appeared in the show as a shout-out to TWoP), House, 24, Star Trek, America's Next Top Model, Six Feet Under, Gilmore Girls, Alias (II) (III) (IV), Prison Break, Desperate Housewives, and many more, including ones I just didn't understand.
The copy is coy as well, winking to those in the know, which is either clever marketing or clever legal advice, possibly both. E.g., the second BSG shirt says: "A toaster is capable of a lot more than you might think at first glance. Even more than toasting your bread, actually. Treat it poorly, and it will find a way to collude with other toasters to overthrow humanity. But treat it well, and...well, you and that little old toaster might actually find love. It's not a love that either of your communities will sanction or even understand, but you know best: that seemingly simple appliance loves you from the coils of its heart."
So, assuming that the relevant consumers (TWoP-using fans) will understand the references -- which is why they buy the shirts in the first place, so it seems pretty likely -- are the images and words serving as indicators of source? Should trademark owners have the right to authorize such merchandise? I think not -- and I doubt that consumers regularly perceive references as indications of source or sponsorship, even if they've been successfully trained to expect authorization for ordinary repetitions of a primary mark on promotional goods like T-shirts. But I'm not sure trademark use can help in that analysis.
(The image at the top of the post appeared on a shirt with the words "Look upward," which were used in the opening voiceover on later seasons of Farscape, the brilliant SF show; the image is of the Farscape module against a wormhole. I wish I'd ordered that shirt when I had the chance.)