Yes it does. Running dual 5870s on a 3x1 display @ 3840x1080. The second card did nothing the whole time. crossfire does not seem to be in use (yet?). No noticeable lag at all except when I was in zones where everyone else experienced it.
Worked great. I just got my hands on a third 24 inch monitor and will try out a large resolution at 5760x1080 when the servers come back up.
Edit* All graphics settings were on their highest setting and shadows on.
#12565