You can't just access any file in a non-jailbroken iPhone.
You can access data files for your own programs during development
using Xcode's Organizer, go to Devices -> your iphone -> Summary
-> Applications -> your app, click on the triangle thingy to show
the Application Data, which you can download.
Otherwise use something like Airsharing (see moogs' answer which
came as I was writing this) to upload&download a bunch of files.