I am pretty sure it is. Most of the banking websites have good security (128 bit) or better. The blackberry can handle that and it is encrypted while banking online just as doing it on your computer.
I use my banks mobile page to log in. By going to the bank of america site, it recognizes I am using a mobile phone and loads the mobile page automatically.
Posted from my CrackBerry at wapforums.crackberry.com
I would be more concerned of using WiFI at your local starbucks. Cell connections are encrypted anyways with 42 second rotating algorithms. Hence the inability to listen in on other peoples conversations.