A quick search of the Android Market reveals approximately 700 distinct "Bank", "Credit Union" and "FCU" (common acronym for Federal Credit Union) apps. Most of them are specific to a particular bank.
Using data compiled from the feds, this URL pegs the total number of banks in the US at over 9,400, which would mean most banks do not have a smartphone app. I know mine doesn't.
Regardless, there would be no reason to assume that a smartphone app for banking is any less secure than an online bank's website.