New app on the Android Market today, the Roam Override Widget. The widget allows users of Qualcomm based CDMA phones to manually force their phone into a roaming mode. This is particularly useful for users who occasionally experience a weak, unreliable signal, but the phone will not roam to the stronger network. Unlike other options, this widget does not require root access.
Tested on the LG Optimus S, but should work on others. Give it a try and let us know your experience! Most custom ROMs do not include the software stack required for this to work. This app is targeting users who do not root or modify their ROM.