Paypal and Square both have a headphone port dongle for accepting credit cards.
https://www.paypal.com/webapps/mpp/credit-card-reader
https://squareup.com/
We already have the hardware demonstrator for accepting magnetic stripe cards http://blog.tehinterweb.com/?p=94 what is needed now is intergration into the API, it seems that Square removed their Android open API but Paypal seems to have a good API if someone wanted to develop an app.

Bitcoin did the first mobile to mobile transfer between two N900's but there is no nice QT app that I know of.

Proper bitcoin: bitcoin URI handling would be nice. I'd definitely add support for that to my applications (so that users won't need to copy-paste the Bitcoin address manually to donate), if it becomes available.

I was thinking it would be cool to have a physical credit card swipe using a Square dongle that dumps the card info to say mtgox and buys that amount of bitcoin and also manages send/receive and maybe multiple backup of your bitcoin. Even cooler if it can interface with a few major banks or (shiver) paypal so you could also downconvert to dollars, euro, etc.
The geekiest phone really should have the geekiest payment app, the existing app gives a good example. https://mtgox.com/mobile

the API
https://en.bitcoin.it/wiki/MtGox/API

here is the python to plug mtgox

from urllib import urlencode
import urllib2
import time
from hashlib import sha512
from hmac import HMAC
import base64
import json
def get_nonce():
return int(time.time()*100000)

def sign_data(secret, data):
return base64.b64encode(str(HMAC(secret, data, sha512).digest()))

class requester:
def __init__(self, auth_key, auth_secret):
self.auth_key = auth_key
self.auth_secret = base64.b64decode(auth_secret)

def build_query(self, req={}):
req["nonce"] = get_nonce()
post_data = urlencode(req)
headers = {}
headers["User-Agent"] = "GoxApi"
headers["Rest-Key"] = self.auth_key
headers["Rest-Sign"] = sign_data(self.auth_secret, post_data)
return (post_data, headers)

def perform(self, path, args):
data, headers = self.build_query(args)
req = urllib2.Request("https://mtgox.com/api/0/"+path, data, headers)
res = urllib2.urlopen(req, data)
return json.load(res)

I am coming back to this thread as I think it is both important and for me topical.
I recently purchased an item from another TMO user and after around five month of hell things are in shape, I have lost over $50 on a $250 deal thanks to Paypal and he lost more and had his account frozen due to lazy incompetent Paypal staff, hopefully it will be freed up soon.

The N900 was the first mobile device to do a Bitcoin transaction, the binary is available now for command line on ARM devices.
A user friendly bitcoin wallet app with functionality with one or more of the bitcoin exchanges makes it a real way to send money internationally.

I asume those who have the option can use a web browser to do bank transfer.