1975 Aluminum Cent

In all likelihood this is a measure to prevent pilferage. Since at least some of these coins left the mint, there was an obvious risk for diversion. Making them illegal not only prevents theft, but also decreases the value of any coins that may have been taken. Besides, who wants to count all 1.5+ million pennies to make sure one isn't missing.

USD/LB Cash 3m 15m
Aluminum 1.3327 1.3544 1.3848


from: http://www.metalprices.com/

I'd never heard of 1975 AL cents. The story I'd always heard was that 11 of the 1974 AL cents were given to various members of Congress and that 1 of them was returned, leaving 10 somewhere out there. The mint probably still claims ownership of any AL pattern cents. I believe the fed gov't recently confiscated one of the 1933 Roman Numeral 20 gold pieces, so I would definately not let them know where yours is.
On edit: The WIKIPedia entry said that about ten are believed to be "out there". If that's correct, then I'm guessing that an authentic one would be worth a whole lot of money (on the black market of course) I don't think it would be a stretch to say one is worth $10,000 or possibly a whole lot more.