As of yet you can not actually sync document to your iPhone. If you email them to yourself you can view them but currently editing is not supported (without jailbreaking the iPhone). An alternative is evernote.com you can add you documents to your evernote - with a desktop application - account and sync them with the iPhone through an iPhone application. Still editing is not available, possibley in a later release of the iPhone software or through a thirdparty application in the app store.
Evernote:
http://www.evernote.com
iPhone app:
http://phobos.apple.com/WebObjects/MZStore.woa/wa/viewSoftware?id=281796108&mt=8
