I've worked extensively with banks as a "vendor" who offers security equipment, but never actually in a bank. There are a variety of different jobs that fall into the IT and security areas. These include: internal systems, retails transactions (PIN, credit), wholesale transactions (EDI, etc.), and more recently customer internet systems. Don't forget that IT security in a bank also has to closely interact with the physical-security and auditing fields. Indeed, taking some financial security auditing courses is a great way to improve your chances of getting a job (I'm thinking of something like http://yourcpas.com/security.asp).
There is a wide variety of banks, and the opportunities vary. But there are two general types of large banks (at the risk of over-simplification); those that are waiting to be bought out and those that acquire other banks.
The waiting-to-be-bought bank is typically run on very slim margins, and tends to skimp on the infrastructure. Although I'm sure they provide some good jobs, you had better enjoy figuring out how to get routine things done with a minimum of expense. This is good for people who don't want to change much (kind of a contradiction for many in the IT field).
Since I got in to my career for the chance to be creative, I would tend to look for a bank that tends to acquire other banks. They also tend to look for innovative programs that can help them meet their goals, and there are better opportunities for lateral-changes and promotion. You may still have to deal with negatives, too much to do with too little money, but at least you are trying new things and there is good opportunity. In the late '90s both First Union Bank Wells Fargo were good examples of what I'm talking about (no guarantees they still are).
Small banks and credit unions vary, because they often tend to outsource a lot of their IT. But you can learn a lot, if you manage to get in. This might be a good place to start, although I think it is harder to find jobs in the smaller institutions.
