There is a user report from Anthony Gravett on macintouch.com that Apple’s Thunderbolt to Firewire adaptor works with an audio interface.

I have just received Apple’s new Thunderbolt-to-Firewire adapter today and can confirm that it works with my Firewire-only audio interface, an Echo AudioFire2, attached to a MacBook Air 11” (mid-2012).

The adapter appears to add a ‘phantom’ Firewire 800 port to the entire system ” visible in System Report ” so even more-specialized Firewire peripherals should function fine. I am not sure if it provides the same amount of bus power as “real” FW ports, though it provided sufficient power to the AudioFire2.

Resource usage seems to be quite small ” similar to using a native FW port when used for 96 kHz audio; the only sidebar issue is that both ends of the adapter get quite warm during operation (indicating perhaps that there are electronics in both the FW and TB ends).

Several people have responded explaining that this shouldn’t be too surprising as the non-display portion of a Thunderbolt port is effectively a PCI Express slot, so any device attached to it will have to have circuitry similar to a PCIe card. That means that driver-wise, they’re pretty much the same as native. Never the less it is good to hear it is working.