Is the job you are interviewing for in the same line of sales, or different? If is different, then it will be easier to address. Not all jobs are right for everyone--even great employees. I'm guessing you don't think they have contacted your former employer yet. Did you have a good relationship with any of your bosses, even though it didn't really work out?
As a former employer to many employees in the past, I always looked at the employee as a person--trying to make it. If they didn't lie, cheat, or steal, and after they were gone, I always tried to help them find decent employment by giving a decent reference. If they were not great, I didn't get into detail. I would emphasize their good qualities, and if asked if I would re-hire them, I would state that they were not right for my particular job, but was sure there were other jobs they would be fine at.
Also, if they were the worst employee ever (this does not relate to you, Halidon) and I was contacted by a direct competitor for a reference--I gave them a glowing reference.
As to whether come clean or leave it lurking...that is a tough call. First I would try to find someone at the old place who will help you by giving you a decent reference. For ME, it would depend on the report I had with the new company. I would be inclined to come clean if you could present a good counter reason to hire you. They may respect and admire you for that.
In fairness, (and brutal honesty) however, I have been hired for jobs where most of my resume was fabricated--and pulled it off without a hitch. Sorry for the ambiguity. Those are just my honest thoughts and opinions. Good luck.