"No comment" is what you'd use if you didn't want to quash rumors running rampant, whether or not they were remotely true or not. It should be noted that people will leap right to the conclusion you ARE doing something of what you're being asked if you come back with "no comment". It's not like you're pleading the Fifth when you do that- inferences can and will be made and you've inadvertently leaked information after a fashion. No, you didn't directly say anything, but you did still give out something all the same.
"Not working on it at this time" is what you'd use if you had been doing something or had it in the plans, but wanted to quash the rumors running about without promising anything. It's int he present tense, yes, but it's NOTHING definitive like you're making it out to be. It could be that they've got something but aren't working on it right now. It could be that they've got just prototype stuff (based on what we DO know right at the moment...) and they're holding off on working on it. And the list goes on and on. Nothing at all definitive about the statement other than they're not working on it right now.
If you want definitive, you'd have to say "No, we're not working on it and have no plans in the forseeable future either," or something like it for the negative and "Yes, we're working on it and plan on shipping it soon," for the positive.