With iTunes 7.6, you can now manually manage music on your iPhone. You can't, however, listen to music stored on your iPhone on your computer. A new release of ecamm's iPhoneDrive makes that possible -- if you're on a Mac.
iPhoneDrive a tool created for storing files on the iPhone as if it were a removable drive, much like the iPod's disk mode function (which is missing, by default, from the iPhone). It can copy music and other media to or from the iPhone or iPod Touch.
Version 1.4 allows you to play music stored on the iPhone or iPod Touch. The new release also lets you access (and copy) call logs, SMS messages, photos from the camera.
A 7-day trial version of the $20 application is available.