While I like open source and Free ware there are some really nasty malwares going round on ANDROID phone apps - one of the latest and nastiest is a little nasty that unknown to the subscriber it sends PREMIUM RATE text messages. The subscriber then gets a real shock when the next bill comes in.
Android apps can often be freely downloaded from locations other than google's App store so no decent control is exercised over these.
Having an open system like this obviously has problems.
Windows 8 could provide some security on the next generation of smart phones.
I don't think I'd use a mobile phone for online Banking -- no problem with a PC however.