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.
Michael Larabel is the principal author of Phoronix.com and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 10,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated benchmarking software. He can be followed via Twitter or contacted via MichaelLarabel.com.