If that doesn't work you can use this, which was actually provided on another roam only thread:
->dial: ##PRT#
->enter your SPC/MSL
->click view info
->Modem Settings
->click & hold preferred serving system
->edit will appear click on it and choose roaming only
->click ok
-> it will ask to reboot, click yes, then ok on the little box
And it will restart in roaming mode.
Might not be the most practical solution, but a couple of the guys and me weren't able to roam only using the cab or the manual registry edits. I actually have gotten used to the process (you have to do the same to put it back on automatic or sprint only). Good luck!
EDIT: here's the thread I was referring 2:
http://forum.ppcgeeks.com/showthread...t=roaming+only
check post #56