There are many different opinions when it comes to this topic. In highly competitive niche industries the best talent is typically gainfully employed by your top one or two competitors. Why settle for table scraps (active applicants) when you can go after "all the fixins"? Is it more acceptable for a Corporate HR department to put this request into the hands of a Third Party recruiter? Is it unethical across the board?
Personally, I think it is the best resource for applicable talent in a niche industry. As long as your direct competitor is not a business partner in any form, rock it out!!!! You will find that it is beneficial to your organization because it will reduce the rampup time needed for a lesser candidate or one from a "similar" industry and also may provide you some "off the record" tips. Of course all of this can only be possible if you can get around the non-compete clause.