It's great to see how ThinkGeek uses April Fool's day as a way to gauge customer interest. They did this before with their Tauntaun sleeping bags which they produced/sold after a lot of positive feedback to their April 1st joke.
Also- While the orig. idea was by ThinkGeek, the production isn't by them for this product, it's a third-party.
I don't really think the April fools stuff is being explicitly used as test marketing, btw, though I think it's a neat side effect. Those guys just really like April fools stuff.