With the most recent Catalyst 8.9 Linux driver release there is support for MultiView on FireGL and FirePRO graphics cards. This allows the user to use multiple graphics cards together in order to build a single X server that spans all of these displays. With some motherboards such as the ASUS P5E64 WS Professional having four PCI Express x16 slots, you can have four graphics cards and if each one provides two DVI ports you then can have yourself an eight-monitor setup. Each monitor can be configured through the AMD Catalyst Control Center Linux Edition.
When using MultiView, yes, you still have OpenGL acceleration on all of the monitors. Someone has posted a YouTube video that highlights this with the Catalyst 8.9 driver, Ubuntu 8.04, and four ATI FireGL V5600 graphics cards. This ten minute view shows the eight-monitor ATI Linux setup running through a CVS snapshot of the open-source FlightGear program, which supports multiple monitors quite well.