The current TL;DR on Thunderbolt DMA attacks is that the VT-d IOMMU is doing its job and Ivy-Bridge (2012 and later hardware) Macs running OS X >= 10.8.2 are not vulnerable to the easy direct-write-to-memory-style attacks we saw popularized with firewire.
While Inception claims to work with Thunderbolt, it's really only a firewire attack, so you need a Thunderbolt to firewire converter and it's subject to the same limitations as normal firewire, as described at the end of this post.
There was a great 2013 Blackhat talk by Russ Sevinsky that covered lots of chip-level reverse engineering for Thunderbolt, but ultimately he didn't come up with an attack (excellent description of the reverse engineering process though). More recently snare described how to set up an attack on thunderbolt with an FPGA board connected to a mac via a Thunderbolt-PCIe device. But the IOMMU foiled his efforts on modern hardware. Snare says he's working on trying to bypass VT-d, so there may be interesting developments in the future.
For now you should probably still be more worried about snare's other work using PCIe option ROMs as a bootkit.