On the back end you’ll want Gig-E between VOIP servers. To the desktop most users don’t need Gig-E. In my shop we use Gig-E to the VOIP ports so that we don’t have to have two sets of network switches.
Like MrDenny mentioned, GigE to your phones isn’t ABSOLUTLEY necessary. Since the phones are in essence switches themselves, 100mbps on different voice/data VLANs is more than sufficient for your “typical” users..
However, Gigabit to the desktop is still really sweet… 🙂