You're not the only one, it seems there is a thread over at sound on sound, and many customers complaining they can't get the Focusrite Saffire Pro 40 to work.
I bought one for christmas, and it won't work on my macbook pro.
The installation CD has written on it "Mac Universal Binary, Windows XP, Windows Vista", but Focusrite also accidentally forgot to put the mac drivers on the CDs (how does that get past beta test?), so all the CDs have been shipped without mac drivers. No biggie, as you can download from their website, but it says a lot about Focusrite's whole end to end testing process.
I don't know whats wrong with my saffire pro 40, but not only are there rumours of faulty units, and faulty firewire leads, the drivers don't even seem to install correctly in OS X.
Focusrite support is closed until 5th Jan (?!), and they say it will be limited after that as they catch up from the christmas period.
The sickening thing is, I actually made the effort to call focusrite before purchasing this product, to ask if there were ANY incompatibilities or faults or problems with the units, and they said there are no problems.
Overall, this is an interface to stear clear of, certainly for the moment.
Like everyone else, I've been left out on a lurch for weeks without a working interface and my opinion of Focusrite is going down. Like the rest, I'll be calling up focusrite first thing tomorrow.