Parody is the safest route I take when designing t-shirts or prints for IP's I don't have the license for. It's a gray area of US law, but usually a parody of something (has a clear underlying message or commentary) is fair use and won't be challenged (think mad magazine, etc). Like Olly's Samus design is a parody on Mega Man, and doesn't use licensed art or trademarked wording. Some companies love fans creating and selling art, promoting/marketing their IP while building a community, while others, usually larger companies, see it as a loss of profits and hunt down even the smallest fry. It's a tricky business, but if companies made it easier to apply for an IP license, I for one would gladly take a loss in profit to make legit licensed work. Funny enough, some of my parody work has gotten me the connections to produce licensed work for big IP's. I've created licensed Wreck-It Ralph, Avengers, and Star Wars pixel designs for example.
