lost access to computer iphone syncs to


Sorry if this question has been asked before, I searched and searched but couldn't find anything similar. I can't believe it's unique though, perhaps I'm not using the most obvious search terms.

The computer that I sync my iphone to was stolen. My new one won't let me sync to it as it realises the phone used to sync with another! It will only let me erase the contents of the phone and start over.

How can I tell the new computer "Hey, you're my new computer, just start with what's on the phone already and we'll go from there"?
I think your goal is to transfer all your stuff from the iPhone first.
If you have items on your iPhone that you don't want to lose, and exist only on the iPhone, then the best method would be to sync FROM the iPhone to the Mac that you want to use now. There's a variety of different apps that will do that.
DiskAid is one: http://www.macupdate.com/app/mac/27830/diskaid
PhoneView: http://www.macupdate.com/app/mac/25347/phoneview
iPod.iTunes: http://www.macupdate.com/app/mac/7546/ipod.itunes
iExplorer: http://www.macupdate.com/app/mac/31722/iexplorer

I don't know which would be best, as it depends on what data you need to retrieve from your iPhone.