I subscribe to the PC Pro magazine email newsletter and recently read an article there on this subject. Unfortunately I've been unable to find it on their website but it said that with the number of apps increasing and the number of people keeping details of contacts etc and doing their banking online via smartphones there is a need for anti-virus software to be installed.
As the apps on the Android Market aren't as rigorously controlled as those available with Apple there will be rogue apps which infiltrate the phone and act as trojans etc to disclose passwords/usernames and the writers of these apps may be able to hack into a users personal bank accounts IF these are accessed via the phone and IF a malicious app is installed.
Basically you've got a mini computer in your pocket. You have anti-virus on your laptop or desktop and should take the same precaution with your smartphone.