Two iTunes installs pointing at the same NAS share
My solution for that is having only one computer managing the library. First it was a PC running iTunes for Windows. Then, I moved to Ubuntu and started using Banshee for both managing the library and keeping my iPod updated. Finally, I have a Mac Book Pro and I have moved back to iTunes.

The Linux server, running both Samba and Netatalk, acts also as a mediacenter (with XBMC) that shares music also over uPNP, so I can access and play music from virtually any network device, even from the Internet.

In the past I've tried other schemas involving iTunes and network filesystems and it always turned into a headache.