Banking using the cellphone is the way to go but cellphones dont offer any security. WAP is insecure, SMS is insescure, but WML over WIG is secure. Its very easy to spoof the originator of an SMS. USSD is even more insecure and I dont think the public are being made aware of how insecure their data is.
The only way to secure is using a SSL stack but that takes the handset out of the reach of average joe.