Coding of driver has commenced but there is no ETA at this point.
Update, 6 Jan 2011: For those following along at home I thought a brief update is in order. Work on the driver continues, but has been held up for 3-4 months with a significant problem associated with the audio streaming. As of just before Christmas I think this is now fixed, but I need to do further testing to confirm this. Assuming the problem is truly solved there are some minor cleanups needed, plus the resolution of a minor issue regarding audio streaming. Once done I expect the driver will be in sufficient shape for people to start testing it with their own fireface-400 setups. Due to preparation and travel associated with Linux.conf.au 2011 it's probably going to be early February before I get a chance to do this, but I'm hopeful that a workable driver for the fireface-400 should emerge pretty quickly after that.
As for the fireface-800, I intend to focus on that once the ff400 is operating in a useful way. The ff400 was first because it's the one more people are likely to have and it physically fitted easier in my development space. While the ff800 does differ from the ff400 in several significant ways I do not expect it will take too long to get basic functionality going once I get onto it.
Finally, to all the ff400/800 owners who have expressed an interest in the drive: thanks for the interest and the patience. I hope to reward you with something workable soon.
Update, 17 April 2011: a limited preliminary driver is now ready for wider testing. Please refer to the main announcement for the details.
Update, 27 April 2012: the driver is approaching a state where it should be usable for most people now. There are a few known outstanding issues and these are being worked on (most notably, an occasional failure to cleanly start audio streaming). "Support status" has been changed to reflect this. Test reports and suggestions are welcome. Note that support for the MIDI ports is not currently implemented.