Insanely awesome iPhone hacker and developer "Core" just finished implementing AFP for the iPhone and iPod touch. This software connects your iPhone (or touch) to your computer using AFP, the AppleTalk Filing Protocol. Your iPhone shows up on your desktop as a disk with full read/write access.
The AFP server is brand new, so it is not yet available in Installer.app. To install by hand, use sftp to copy the tar file into /opt/iphone. Extract the archive on your iPhone or touch--the tar archive program is part of the BSD program; use tar xvf name-of-archive.tar--and run /opt/iphone/afp/startserver.sh &. The ampersand lets the program run in the background. (You will need to restart it after reboots.)
Once installed and running, go to Finder. Choose Go > Connect To Server, and enter the afp address for your iPhone, in my case afp://192.168.0.111. Just use the afp:// prefix with the local IP address of your iPhone. Enter your user id (root) and password (alpine) and your iPhone or iPod appears in the sources list for your Finder windows.
Once your iPhone learns how to add itself as a disk, you'll start wondering how you ever lived without this. To add new applications, just drop them into the Applications folder. To back-up your personal data, just copy /var/root/Library. This is just an awesome accomplishment. Thank you Core!
Hopefully soon, we'll see not only an Installer.app version but one with a controllable launch daemon that will let you enable and disable the service as desired.
Update: If you're installing by hand, make sure you use the version for Registered Users. It's the one from November 10th. Don't use the earlier one that only allows guest mode.