You'll soon be able to buy ArduPilot already made, but for those of you who would rather build their own, here are the necessary parts:
You can buy the boards here.
Eagle (PCB and schematic) files are here.
A pdf of the schematic is here.
Once you've made the board, instructions on setting the fuses and loading the bootloader are here.
A few notes on building the board: Aside from all the surface-mount components below, the only through-hole components you need to solder on are a 5-pin breakaway connector for the FTDI port (first five pins next to the GPS connector), the two 2x3 ICSP connectors, two 3-pin connectors for channel 1 & 2 output, one 3-pin connector for the power select and three pigtails (female-to-female servo connectors cut in half) for CTRL and channel 1 & 2 in. The other two channels (3&4) are not currently used. IMPORTANT: the power selector pins must have a jumper cap placed on two of the pins. Use the two closest to the servo-out ports if you're going to power the board from the RC receiver via the RC-in cables (that's what I've done in the picture above), or use the other two if you want to power the board with a separate regulated 5v power source. If you don't put the jumper cap on these pins, the board will have no power!