The US government is prohibited from asking questions about religion, therefore this has become an urban legend floating around the pagan circles of the net.
The e-mail you may receive may look like the following, or at least contain similar meaning:
This is REALLY important - forward it to every US Pagan you know!
2000 is a census year, and we will all be getting that lovely little form in the mail asking us loads of questions. One of those questions asks about religion. A point stressed at the workshop was the urge every Pagan you know -- closeted or not, regardless of how they classify themselves -- to write in PAGAN.
Why Pagan rather than all the other words we use to define ourselves?
Quite simply: numbers. If some of us write Pagan, others Wiccan, earth religion, etc, etc, what is going to happen is we are all going to end up under OTHER, and that doesn't do us any good. If we all write in Pagan, well then, our numbers will be significant to the census people and more importantly, we will be COUNTED. Imagine how awesome it would be to find out on the official census report that 10% (or whatever percent) of the US population considers themselves Pagan! Remember, the census is anonymous, so we don't have to worry about outing ourselves!
Numbers = recognition and recognition = legitimacy. So, in a nutshell, join the campaign, pass on the word to write in "Pagan" on the US 2000 census. Stand up and be counted!