The Attorney General in New Mexico issued an opinion, which while it doesn't ban horse slaughter in the state, has certainly put a high standard in place which makes it highly unlikely that the proposed slaughterhouse in Roswell will be profitable. According the AG, Gary King, horse meat is a tainted product so even though it will not be consumed in the state, it is illegal to manufacture it under state law.
Under the AG's opinion, a horse may be slaughtered for human consumption only if the horse's drug pedigree is known before slaughter to ensure the meat is not contaminated by drugs not intended for use in animals intended for human consumption. This means it is not possible to slaughter race horses and likely many pleasure horses as people don't tend to keep accurate records.
If the opinion stands, unless the company which owns the slaughterhouse decides to raise their own horses, the number of horses which can be slaughtered will be so small, the plant will not be able to operate profitably. Hence, unless Valley Meat Co. decides to challenge the Attorney General's opinion and risk legal liability, the plant will not be able to operate as a slaughterhouse for horses.
That being said, don't be surprised if we haven't heard the last of this case.