If you want more than just file transfer, Box.net has an iPhone client that lets you share your documents in its online storage space. You can also upload photos from your iPhone and add collaborators by folder too.
I like Box because your files can also be seen on your LinkedIn page and you can incorporate a vast library of "Open Box" add-on applications. However, Box isn't perfect -- these add-in applications aren't included in the iPhone app; you have to make use of the Web client. And the iPhone app is missing some of the more interesting controls that are found in the Box Web client, such as being able to create a unique URL for ad-hoc file sharing.
David Strom is a veteran technology journalist, speaker and former IT manager. He has written two books on computing and thousands of articles. His blog can be found at strominator.com.