I have a iPhone 3Gs and I had it synced to my mac, then my mac broke, once I have reinstalled mac OSX it had wiped all my data and my time machine broke too. So not very happy. But when I go on iTunes to make my iPhone manually manage music it asks my 'Erase Data' or cancel.
This is not helpful but any help would be appreciated.
I have not had that problem with my iPhone, but it is very similar to one i have with my ipod, i have two user accounts on one machine, even thogh i have selected manually manage, if i try to download from the 'second' account, it offers to wipe the iPod.
good luck getting an answer.
You can use a program called Irip for pc, i am not sure if they have a mac version. Their are numerous programs that can to that. Remember each ipod and iphone can only be linked to one unique computer in terms of the sync feature. So you have to manually pull what you have from your iphone for that to work.