Why would we want to do this?
I recently mentioned the Arduino for the MultiWiiCopter project. Although the cheaper Arduino Pro Mini (15 USD) is very well suited for this project I did had some issues with the programmability. You need a separate USB to serial adapter costing around 15 USD. Adding the cost of that doesn't make the Pro Mini an affordable option anymore.
You could go for another Arduino format. Like the Arduino Nano that comes with a mini USB connection or the Arduino Uno having a proper USB connection. The last generation Arduino's don't even require a driver for the USB chipset anymore.
My suggestion? Get an Arduino Pro Mini for this project and spend the money of the USB to serial adapter on another Arduino Uno. You can use these to program the Pro Mini and you then have a spare Arduino platform.
How to program a Pro Mini with another Arduino
Here is a simple way to program an Arduino Pro Mini without the use of a USB/FTDI programmer. All you need is another Arduino! Simply pull the cpu out of the board, and wire the following pins between the Arduino and Arduino Mini
RX -> RX
TX -> TX
+5v (or +3.3v) -> VCC
GND -> GND
Reset -> GRN (Reset/DTR)
That's it! Plug it in an program You could also use this method to add serial/usb communications to your Pro Mini for debugging purposes (just don't wire up VCC if it's being powered from another source).
I used an Arduino Serial, but you could use a USB Arduino too.
It's pretty straightforward, but here are pictures anyways: