not very helpful replies so far!
I'd say that the banks who made the apps wouldn't allow an insecure app to be made - as fraud costs them millions every year, expect pretty good encryption, atleast 128k.
I'd say you were pretty much safe, just aslong as your phone isn't jailbroken (adds a chance that a dodgy app could intercept the password)
Example form the citi app:
"ENCRYPTION — Your account is protected by 128-bit encryption, just as it is when you use Citibank Online.
PASSWORD PROTECTION — Each time you use Citi Mobile, you enter a 6-digit telephone access code, which you create when you enroll in Citi Mobile.
PRIVACY PROTECTION — The only personal information stored on your iPhone is the last 4 digits of your ATM/Debit card — the same information typically found on a Debit Card or ATM transaction record.
INSTANT DEACTIVATION — If your iPhone is lost or stolen, call 1-888-214-0036 — we can deactivate Citi Mobile instantly."