Raybay, why do you believe this can't be done? I'll be the first to admit that I'm always skeptical of these stories because they often reek of sensationalized oversimplification... But there's plenty of proof this legitimate:
http://news.cnet.com/8301-27080_3-20013246-245.html
Once you've rooted a computer, pretty much anything is possible. Even trusted, secure connections can't be "trusted" anymore because the computer you're using itself is not trustworthy.
Programmatically farming usernames/passwords and site they are used on from most browsers is probably a pretty simple thing. In Firefox, you can view your saved passwords in plain text and there are various utilities to 'view' your passwords with other browsers like IE. This is all very much a reminder NOT to have your browser save your password.
Even a keylogger would be good enough if a pair of human eyes. Maybe the bank transfers themselves happen by hand, but the information collected is done programmatically.
There are plenty of ways to manipulate browsers and inject/replace HTML. Swapping out customer's real balances can be just a matter of getting a simple add-on/plugin/extension installed outside of your browser. You'd need to be familiar with banking websites, but how hard is that? Even if it affects only a handful of the largest banks, you've probably got 90% of everyone who banks online.
And lastly, your connection simply isn't secure anymore. Banking sites are encrypted end-to-end, but when you control one of those ends, you can expose what's going on to your delight because even at the very least, injecting a MITM is well within your power. Having root means any of the above can be done.
I see no reason why this can't be done. It contains a few steps and things have to work properly, but that's why this is being touted as a 'sophisticated' virus.