Nope, doesn't exist. Probably never will. Just buy Windows, install it on your Mac, and get the Windows version.
Keep in mind that unless you have a GeForce 8800-class card or better, you won't be able to run the game very well at all. If you have an iMac with a Radeon HD2600 or a MacBook Pro with a GeForce 8600M or 9600M, you'll probably see average framerates in the low 20s (i.e. playable, but only just so) at low settings and low resolution.