I don't get a crap ton of recruiter mail. (Yay?) I'm in a good gig now, so I don't really respond to the ones that I do get. Sometimes, if I feel that they're not too slimy, I'll respond by saying "I'm OK now, I'll keep you in mind down the road if I ever need anything," and file away their contact info in the event my current gig goes south. My life is busy enough that I'd probably be annoyed at the presumption I've got time to deal with a work sample test on top of dealing with the spam in the first place. However...
I'm definitely one of those people described in XKCD's nerd sniping strip.[0] If the sample problem actually flicked that little nerve in my brain, you might get a response from me just because I couldn't help myself. But here's the kicker. You'd only get my solution as long as I could email it directly to you instead of the recruiter. At that point, even if I'm not interested in working for you now the sample problem you wrote has sparked some interest. Now, I'm interested in striking up a relationship with that might not amount to much at the moment, but may prove fruitful in the future. If we've gotten this far, I'm not interested in dealing with the recruiter any more.