iTunes won't recognize iPhone backup file

I have a backup file from my iPhone 3gs that is not being recognized by itunes for some reason. I recently purchased a new iPhone 4 so I need to transfer all the stuff. It doesn't show up as an option but my girlfriend's backup does.

Does anyone have any idea why this is and how I can resolve this?

Thanks!

Does your girlfriend also have an iPhone 4??

The reason I'm asking is that part of me thinks that when you do a backup, iTunes may just do a straight dump of the phone, meaning that every byte has to be in a certain place. And since the iPhone 4 and iPhone 3GS are different models, that means they don't line up properly, know what I mean? Basically, round peg in a square hole...

I could be wrong, though.

Did this copy of iTunes make the backup or have you transferred the file from somewhere else? Same version of iTunes?