The applications like My3g and many others which are used to trick the iPhone to use 3G by pretending it is running on WiFi to allow applications Apple and ATT do not won't to be using 3G. All applications which perform VoIP, SIP, Skype are approved by Apple Store when they enforce the usage of 3G only, Otherwise the application won't run if there is no WiFi connection. Therefore, that feature is useless for the Nokia N900 since it does not have those stupid restrictions.

Enjoy !