What really surprises me about the idea of 'banking via twitter' is how the originating bank got this concept past their internal compliance officer/team/department.
I just came off of a 6-month stint at an up-and-coming regional bank. While there, I learned a couple of really interesting lessons about banking in general:
1. Absolutely every breath they take and every move they make (rock on, Police) is filtered through federal and state regulatory compliance.
2. To my surprise, most non-national banks think nothing of throwing money at software solutions with outside vendors and these banks rarely require direct interconnectivity with what is referred to as their 'core' system. This, as it happens, is often an expression of point #1.
So, I say #1 to point out that *someone* familiar with regulatory compliance must have signed off on the Twitter-banking idea. Many here have noted that the communication with a user's accounts is pushed into a private realm at Twitter, but that doesn't sound like an adequate separation to me. 'Private' tweeting or not, it seems to me that most compliance auditors would reel at the mere suggestion of tossing any account information into that electronic pool. They would also likely need to get some kind of compliance statement from Twitter itself to make the bank tweeting product available.
I say point #2 just to say that I'm convinced there's alot of untapped opportunity in banking for hosted applications. ;-)