..."but if the artwork on the shirt taken as a whole is "art"..."
Interesting, I hadn't thought of that. I suppose if someone places 15 different logos on their shirt, it may begin to lead another away from associating the shirt as being a source identifier for those 15 represented companies (though, from time to time, road-race T-shirts wil have several logos on them, showing the sponsors of their race).
Trademark rights are territorial, so if the companies only have a presence in the US, then showing their mark in Germany should not pose a problem; at least until your friends return to the states with the T-shirt on and get held at the border (just joking!). Probably unlikely that the marks you are speaking are strictly US, but thought I'd throw that out.
I wonder if a caption about the marks would remove it a bit more, as speech? "My favorite US Companies!" or " We survived Company Tour 2011!"
Just thinking, there is no commercial use here. I don't recall off the top of my head whether there must be a commercial use of the trademarks before infringement can be found, or after Trademark owner(s) establish ownership of a trademark, if infringement (commercial use or not) is determined strickly by the Polaroid (or other Circuit factors) analysis...
And, just wondering whether they are planning on printing the shirts themselves or going through a T-shirt maker. If the latter, I'm wondering whether they will have problms finding a T-shirt printer that will reproduce several companies Trademarks without written authorizatin from the company. Perhaps?