I have a new macbook pro and would like to transfer the files from my old macbook pro. I need a firewire->thunderbolt cable to do this. I don't really want to pay $30 to buy a cable that I'm going to use once. Will the genius bar do this for me?
As a former Genius, I can say that we were always willing to lend cables to customers for in-store use, but we would not overtly assist with data exercises. Then primary reason is the legality: if Apple gets involved with your data, they become liable for your intellectual property...which can be a legal mess. It's the same reason Apple doesn't offer a guaranteed backup/restore service during a repair.
So, my advice would be book a Genius Bar appointment, explain your situation to a technician, and go from there. The worst they can say is no.
P.S. Unless Apple Retail Store policy has changed, you have a 14-day return policy. I had customers buy a cable, use it for a few days, then return it due to "buyers remorse." ;)
There are many ways how to achieve this - see Apples documentation for file transferring: http://support.apple.com/kb/PH11466
If both MacBooks are at least on OS X Lion (OS X 10.7), I recommend the AirDrop solution.
If you any MacBook is on an older version than OS X 10.7, you may want to copy them over your LAN via ethernet or WIFI connection.
However, if you'll be transferring a lot of data, you may want to use an external drive for exchange.