Edit: Minecraft 1.0 has added the ability to turn Pumpkins into four Pumpkin Seeds each, by placing it into the crafting grid.
Currently, the only way to obtain Pumpkin Seeds is to find them in Abandoned Mine Shafts; you can't craft them like you can Melon Seeds. This means they aren't a renewable resource like Melons are. (I think this was intentional by Notch to keep Pumpkins a rare item.)