Yes, technically you can use another provider's OS version as long as you remove the vendor file when installing. (and are willing to re-load all of your personal information since sometimes it will not always bring it over and will force you to "format" the BB RAM, while some versions allow you to upgrade it
I Technically have an 8700c, unlocked, running on T-Mobile re-flashed with the T-Mobile version of the 8700. I couldn't stand the cingular menu system esp since they hid icons that were not built in programs.