It will remain. I had a friend (yeah, that's the ticket) who left a company that had a FAN discount with AT&T, and it remained on her account for almost 2 more years after she left that company with no problems until she removed it to upgrade to the 3G iPhone. Then, she applied the FAN discount of her current company.
AT&T doesn't do any periodic verification to confirm that you're still with the company.
Definitely dishonest and unethical, but not a horrible sin that will blacken my soul for all eternity, methinks.