well if you want it mountable you could try:
http://ecamm.com/mac/iphonedrive/
it's not 100% ideal and i'm not jailbraking myself, i'll wait.
but at least i know the opt is there should i feel the urge.
apple maybe a bit extra cautious about allowing files onto the iPhone incase it opens it up to wide for 'other' stuff to get in and/or out...