iPhone Backups on Network drive
On a Mac, iDevice backups are stored in strangely (seemingly randomly) named subfolders in ~/Library/Application Support/MobileSync/Backup/
This is outside of the ~/Music/iTunes folder (standard Library location on Mac OSX).

You'd be wanting to take a copy of all the files and folders within this folder and put them on your network drive. I've not tested if a restore of these items works following a system reinstall and I suspect that iTunes may do some extra stuff behind the scenes to pair these backup folders and files to the devices that they came from.

Backup location on Windows (Assuming system drive is C:\)Windows XP
C:\Documents and Settings\user\Application Data\Apple Computer\MobileSync\Backup

Windows Vista/Windows 7
C:\Users\user\AppData\Roaming\Apple Computer\MobileSync\Backup

(edit) Just realised that this probably doesn't fully answer your question, however, I've found this discussion which is along the lines of changing where iTunes reads/writes its backups