**Edit**: Minecraft 1.0 has added the ability to turn Pumpkins into four Pumpkin Seeds each, by placing it into the crafting grid.

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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.)