Alright, I'll chime in. Here's the thing. "Funny" has no place in a financial application. It just doesn't. There's nothing funny about finances when there aren't any errors. There dang sure isn't anything funny about finances when there are errors.
So what about informal? Can anybody give me an example of an informal statement that is as unambiguously precise about exactly what it means, as the formal statement that it purports to replace? I'll bet you lunch that you can't.
So I don't think informal messages have any place in a serious financial (or medical) app, either.
EDIT: In fact, even with regard to non-humorous messages in apps, have a look at Designing for People Who Have Better Things To Do With Their Lives by Joel Spolsky.
Informal messages send me the message that you're not just terribly focused and serious. Which is fine if you're working at a hot-dog stand, so long as you don't get so casual that you wipe your nose then handle my food. It's not so fine if you're handling my money.
Think of it another way. Imagine you're a pilot, piloting an airplane equipped with a fancy software-driven glass cockpit. You're busy in that pilot seat. Lots of stuff is going on all at once. Do you want the messages you see on those displays to be colloquial, informal, "cute," if not outright funny? (But funny according to whose local regional sense of humor?) I really, seriously don't think so.
My point is NOT that using a financial application is equivalent to piloting an airplane (it obviously isn't).
My point is that cutesy stuff unavoidably makes you have to think harder about whether the message means what you really think it means. It unavoidably softens focus and distracts.
If I'm using your banking app from home, or at one of your ATM's, I'm not doing it because I enjoy ATM's. I'm doing it because I have something specific I need to get done so I can go somewhere I actually want to be and do something I actually want to do.
I don't want your app to be my buddy.
Informal communication (I'm not talking about casual dress Friday, here) is by definition less serious and therefore somewhat less trustworthy than concise formal communication.
In my humble opinion.