Palm have just released the upgrade for both the Treo 750v and the generic Treo 750. Previously only the Vodafone Treo 750v upgrade was available.
The upgrade will take your device from Windows Mobile 5.0 to Windows Mobile 6 Professional. It's definitely a worthwhile update as it the Treo 750v really felt like a new device to me once I'd upgraded mine.
You can download the upgrade for FREE from the Palm Website HERE