How safe is internet banking using mobile phone ?

I have never owned an internet enabled cell phone ( can't stand the things), but now the wife is showing an interest in something like Blackberry which I suppose is a glorified cell phone ?
Anyway my main concern would be security when banking etc. Kindly elaborate
Also there must be a hell of a lot of scrolling ? The buttons on most of these devices are also too small for me to use reliably if you know what I mean.