You have a couple of options..
The cheap/easy way is to use the XCM Cross Battle Adapter Plus. - http://www.xcm.cc/xcm_cross_battle_adapter_plus.htm
The expensive/time consuming way is to build (or have built for you) a PS360 controller. - http://forums.xbox-scene.com/index.php?showtopic=651946
Which way you'd like to go is really personal preference and how much you feel like spending.
The adapter is cheap, and has a rapid fire option, but you loose the Tilt and 'pressure sensitive' functions of the buttons (not sure about Triggers) and are tethered to the PS3 since it uses a Wired controller.
The PS360 controller is kinda pricey to have made up, or will take you a fair amount of time to build yourself, it also looses the 'pressure sensitive' button function (Triggers do stay Analog) but keeps the Tilt and Wireless of the controller intact since it's a DS3 controller in the 360 shell, and can be made to look a bit more like it belongs with a PS3 system.
A note on the 'pressure sensitive' (Analog) buttons: Since the PS2 days the PS controllers have had Analog buttons, that means they're are not just off or on, but can hit everything in between, like a gas pedal in a car for example. Now this option is nice, but the amount of travel in the buttons on a PS3 controller to actually use this feature is really too small for it to be put to any real use, and I'm not aware of any games that force this option, with the exception of say a racing game where even then it's not really mandatory, just useful. The only buttons that have that kind of travel to put it to use are L2/R2, and on the PS3 controller those buttons feel more like marshmallows and are curved wrong, where as on an XBOX style controller the Triggers are also Analog, have a decent range of motion and are contoured properly.