A financial interest is improper if your job, the amount of your investment, or the particular company in which you invested could--when viewed objectively by another person--influence your actions as an employee.
In the case of a supplier, if you have anything to do, either directly or indirectly, in deciding whether [your company] does business with that company, you should not have any financial interest at all in the company.
Whether it's a good idea from a portfolio diversification perspective is a different matter, but worth taking into account.
So there's no law against it, but if you are an at-will employee, and if your employer views owning stock in a competitor as somehow disloyal, well, you may be out of a job.
posted by ambrosia at 10:13 AM on November 9, 2005