As I said when someone brought this up half a year ago:
Most realists believe in only one observable universe. Solipsists just go one universe farther.
Most baseball fanatics pull for only one team. People who hate baseball just go one team farther.
My favorite quote is from John McCarthy:
"By the way, I count myself as an atheist rather than an agnostic. My criterion for being an atheist is believing that the evidence on the god question is in a similar state to the evidence on the werewolf question. "
I wonder if a polytheist in anciant Greece which believed in one less god (e.g. didn't belive in Cronos, but did believe in the rest of the pantheon) would be concidered an atheist by his contemporaries?