Banking apps not working

Try deleting the apps from the iPhone and iTunes, then re-download and sync them.

You could also check support at the bank's website to see if there are issues.

Which banking apps? Bank Of America, ING Direct, AMEX, Discover, Fidelity - all work for me with 4.2.1 on my 3Gs.

Have you checked AT&T's forums to see if others are having issues with their mobile banking app? It would seem odd to me that other, standalone banking apps work fine with the update and only AT&T's mobile app was fubar'd, but stranger things have happened.

I looked up the Ratings of these apps in the iTunes store and you're not the only one, the 3rd party developer needs to update that's all. There are lots of cranky people (such a shame how angry people get) so I'm sure there will be an update soon.

My banking apps work after the 4.2.1 upgrade, however one of them no longer recognized my device and made me re-enter my login ID. After that, it worked fine.

Not all developers get the new update before it's actually released and not all changes in an update effect all apps, it all depends 🙂