I've asked myself exactly the same thing as I've also bough a Samsung 840 Pro for my MacBook Pro. After some research I've found this post indicating that the 840 Pro's hardware encryption requires TPM support, and that's only found in PC BIOSes, not in Mac's (U)EFI. To be sure, I've asked Samsung support which of the standards "ATA-Security", "Seagate DriveTrust" and "TCG OPAL" are supported by the 840 Pro, and their answer was:
Thank you for contacting Samsung SSD support regarding your inquiry.
In response to your inquiry, the only one of the 3 that the unit
supports is the ATA Security feaure. As for the encryption, the 840
Pro Series SSD only supports AES 256 bit hardware level encryption but
requires the BIOS to be TPM enabled.
So there's no way to enable the 840 Pro's hardware encryption in a Mac.
However, there's also the Crucial M500 which supports TCG's Opal. In conjunction with a special Opal management software like WinMagic's SecureDoc for Mac it sounds as if it's possible to get hardware encryption to work on a Mac.
BTW, note that according to Sophos' support their SafeGuard does support Opal only on Windows, not on Mac OS. Also, McAfee's General Q&A for Opal states
Q: Will Opal drives be supported on Mac OS X?
A: No. Apple currently does not ship their devices with Opal drives so Opal is not supported on Endpoint Encryption for Mac.
But of course that's says nothing about that happens if you just put an Opal drive into a Mac yourself.