The 6990 is not supported under Linux yet, even with the 11.4b hotfix.
Probably will be fairly soon; all other 6900 series cards are.
Not true. I mine happily with multiple 6990s with the catalyst
11.411.3 driver on Linux.
To my surprise, even the older
11.311.2 works, if you write the xorg.conf manually.
That said I do use a
custom CAL miner, as opposed to OpenCL, so perhaps the problems preventing you guys from using the 6990 only affect OpenCL miners.
Edited to fix catalyst version numbers.