Great information on interrogating mounted drives via IOKit in this question. But is there a way to determine which of the devices returned by IOIteratorNext() is the boot drive? Or better yet, might there be a way to get just the boot drive in the iterator returned by IOServiceGetMatchingServices()?
Booting is done from media, not a device per se. Devices have media, media have volumes. I don't believe that volumes are represented in IOKit.
This is probably easiest using Disk Arbitration. Use DADiskCreateFromVolumePath() with the CFURL for /. Then call DADiskCopyDescription() to get the description dictionary. That will include properties of the volume, the media, the device, and even the bus, including IOKit paths if you need them. There's a good chance the information you're looking for is directly in the description dictionary, though.
The NVRAM information cited by Mark Setchell is available from IOKit, too, at path IOService:/AppleACPIPlatformExpert/AppleEFIRuntime/AppleEFINVRAM. There's a property efi-boot-device. Its value is a property list including a service matching dictionary. As you can see, it looks for an entry with provider class of IOMedia whose UUID property is a certain UUID.