I am a different attorney, and I have a very different answer for you. I believe that my colleague may have misunderstood your question.
An umpire, or other sport officiant would fall within the scope of Business Activity Code: "812990 - All other personal services."
An independent contractor would report income either on a Schedule C -- assuming that your activity has a "profit motive," i.e. you intend to earn income from personal services rendered.
If your purpose is simply a hobby, and your income is merely a stipend used to offset your expenses, then you could enter it on your Form 1040, line 21.
Concerning your actual legal status as an employee vs. an independent contractor -- if you have any concern about that, you could file a form SS-8
with the IRS, and the government will determine your status for you.
Hope this helps.
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Edited by socrateaser on 9/25/2010 at 4:03 AM EST