We tried to sync and restore on iPhone 3G last night to no avail:
“Another sync client is already running”
The fix was a permission change to 777 on the “tmp” folder AND its contents.
The easiest way to access /tmp is to use the “Go To Folder” menu item in the Finder’s Go menu and type in /tmp in the dialog box.
Do a Get Info ( Command-I ) on the tmp folder, and change the permissions to read & write for everyone, AND most importantly (read: it won’t fix it without this step), choose “Apply to enclosed items”.
Fixed the iPhone sync issue immediately.