I think I may have portrayed my view wrong, then. I don't have problem with the "bump" thing as a generic interaction. I can see it as a very handy way of doing certain things, like playing a game with a friend. At the moment I'd have to look for all nearby phones(/DSes, etc.) and figure out which one is my friend. Just nodding our phones together seems a great improvement. There are tons of other things I imagine it would be great for.
I do have a problem with its use for transferring money. It just seems pointless. That's what cash is for, and paypal is most useful because I can use it where cash is not an option. I don't see any benefit to using this kind of interaction here.
For it to be useful, we have to be in the same room, I'd have to not have any cash or cheques, or alternatively be required to use paypal. It also has to be something that has to happen soon, or else I could get cash later or use paypal regularly or something rather than going through the effort of us taking out our phones and looking like goofs. We also both have to already have the relevant app installed, with a compatible model of phone. I actually have to make sure of this, too-- I have to ask, "hey do you have that paypal app", etc., which takes more time. This is a situation I do not foresee happening very often at all. There are just so many limiting factors, and it's just so easy to have enough cash in my wallet.