Problem with Target Display Mode

Hi there

I just bought a nice new 2013 27-inch iMac and want to use my old late 2009 27-inch iMac as a second display. The new iMac has 2 Thunderbolt ports and the older one has a MiniDisplay port (the connectors are the same). I bought a Thunderbolt cable to connect the two, as I read that the new iMac can connect to older Macs through the MiniDisplay port. When I hit FN-CMD-F2 or just CMD-F2 nothing happens, and I cannot get the older iMac to serve as a second display. I have shut down and restarted, unplugged all other peripherals, and checked the cables are inserted the right way, etc, etc.

Interestingly, when I use the brand-new Thunderbolt cable to connect my two MacBook Airs (both of which have Thunderbolt ports), I can't get Target Display mode (FN-CMD-F2/CMD-F2) to work either.

So, do I need a male-to-male Mini Display to Mini-Display cable instead, or some other sort of connectors to allow the older iMac to work as a second display?

Thanks

Jon