This tutorial shows you how to build an easily extendable, fully nautomatic and mob-free Watermelon / Pumpkin Farm. No retilling for nfarmland is required.
Note that pumpkin seeds seem to be no longer available as of 1.8.1.

Efficiency
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One module (6 Pistons) yields roughly about 30 watermelon slices or 6 pumpkins per hour.

World Download
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http://www.mediafire.com/?bkj4h16x3oia5i7

Music
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Ashley - Slipstream
Nero - Electron
Intraspekt - New Sensation

Mods/Tools used
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MCEdit
Texturepack: Misa's Realistic

Credits
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- Sethbling for his original BUD design/concept
- Etho for making BUD's so popular
- Ludolik@DeviantArt for the Minecraft Watermelon Girl Pixelart

How it Works
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It uses a BUD by sethbling (slightly modified) next to the two possible ngrow spots. It's important that the BUD (Sticky Piston) is north or south (ie along the x-axis) of the fruit block. That is because those blocks are updated before those above or below (farmland), while those along the z-axis (east/west) are updated last.
As soon as the BUD's Sticky Piston gets updated, it retracts, unpowering a redstone torch which is conencted to another Sticky Piston pulling the farmland down. All this happens within one tick, before the farmland gets a chance to update. Shortly after (one redstone tick), yet another piston gets activated, crushing the Melon/Pumpkin and retracting in time, before the farmland turns back to dirt again because of the piston extension.

Some notes:
- the redstone torch at the rear top isn't needed for the circuit itself, but it prevents the BUD from getting stuck by an unfortunate chunk unload timing or when you exit the game at the wrong time (->Repeater Reboot)
- the wheat seeds aren't necessarily needed, but they allow farmland to be at that spot without having to worry about melons/pumpkins growing there. Farmland around seeds increases the speed at which they grow
- the bottom line might look a bit confusing because of the glass I placed there for the water (in case you don't have water canals right next to the farm)