GetUSB.info has a detailed write-up on how to make your iPhone a storage device via the Wi-Fi network.
What is more economical: Buying a 2GB flash drive or downloading a $0.99 cent application to get 2GBs of free storage on your iPhone? Don’t answer. We know.
MiDisk is one of those great iPhone applications. The app gives you the ability to create Directories on your iPhone for storing files, documents etc. It’s great because you don’t need the iPhone sync app – you can use the iPhone as a mass storage device.