Yesterday, ecamm introduced iPhoneDrive, a utility that lets you use your iPhone disk to store arbitrary data files. I downloaded a copy and after a quick false start and a software update that addressed compatibility issues with my PowerPC G4, I was able to load files onto and recover them from my iPhone drive. Ecamm are the developers who brought us such Mac classics as iGlasses and iChatUSBCam.
Apple seems to have deliberately omitted hard drive storage from the iPhone (probably to protect the underlying OS X files from public scrutiny and hack-cidents). Hard drive storage is a valuable component of any portable media player since it allows you to bring files with you that you'd normally carry on a separate thumb drive.
To deliver your files from one computer to another, you will have to install iPhoneDrive on the receiving computer. I found that file transfer, both onto and off from the iPhone, went smoothly. It took about 5 minutes to transfer a 350 MB 45-minute episode of America's Next Top Model in each direction.
Unfortunately, being the first release, iPhoneDrive has a few flaws that should clear up in later versions. You cannot rename a file once it is on your drive. Also, although you can create a folder hierarchy, you cannot drag items into or out from folders.
I suspect this is just the first of what I hope will be many third party iPhone utilities. You can try out iPhoneDrive for 7 days without restriction. After, it will cost you ten bucks--not a huge price for a feature that many of us wish Apple had built into the iPhone in the first place.