The short answer is, you don't.
Some pre-Thunderbolt iMacs (the 27-inch ones) could serve as dumb DisplayPort displays ("Target Disk Mode"). That was a nice feature because a lot of modern Macs and PCs can generate DisplayPort output.
With Thunderbolt, the rules changed. A Thunderbolt port can drive a DisplayPort display. But if you want to use the iMac as a dumb display, the device driving it has to be a Thunderbolt device (not just a DisplayPort one).
There might be some adapter you could buy to turn the PS3's output into DisplayPort output. But there is no practical way of retrofitting a PS3 to provide Thunderbolt output. Even on a Mac (or on the PCs where Intel is hoping to introduce it), Thunderbolt is, so far, something that has to be integrated into the motherboard.
There is another possibility.
There are devices that connect to a USB port, for the purpose of watching or importing TV and video signals. It might be possible to go from a PlayStation 3, to such a device, to a USB port, and from there, to the screen.
This is a less desirable path than a Target Display connection would be (especially if it forces you to drop down to a NTSC signal or if it introduces noticeable quality degradation to a HDTV one). That's part of why I did not think of it last night. Still, it's a (possible) option.