I think everybody knows it's wrong to rip off someone else's work. But it's also considered flattering for someone to like your work so much that they're driven to incorporate your ideas into their own work. And that's a continuum on which it is very hard to say, "this point here is where it ceases to be flattery and becomes infringement."
Switching to a somewhat different topic (a lateral topic): I've printed T-shirts in the past, and the boldest, simplest designs were usually the easiest to print, and the most aesthetically-pleasing. Text, and a few line-art images worked best. Photos sort of suck, and are expensive and difficult to print. A t-shirt is definitely not a DVD cover.
So perhaps instead of panicking about legal issues, all you interested designers should keep in mind the technical and financial restrictions of the actual t-shirt printing process, and focus on producing a great design within those parameters.
For all its apparent frivolity, Ell's saber-themed design is probably one that won't get anyone sued. It's pretty simple, gets a point across, and is infringement-free!
If I were to make some shirt designs (and I might yet), I think I'd focus on fonts, catchphrases, and very simple drawings hinting at SW themes, rather that out-and-out full colour gradients and photos. I mean, draw a circle with a line across the middle and another small circle above that line, and you've got yourself a Death Star -- but a Death Star that almost certainly won't get you sued!