Hey Captain, although I was quite skeptical a few months ago when I heard about SLi-Hack, I decided to give it a try.
Running a high end AM3 board right now (Crosshair IV Extreme) while running two GTX 580s.
Performance is 100% what you expect on an "SLi-Approved" board. This is a hack that mimics exactly how nVidia enable SLi on motherboards (its just a "software" license now a days) and these boys reverse engineered the code to allow it to work on just about any motherboard.
Here is the official Overclock.net SLi-Hack Thread
Latest version is "Final" Release v 1.0.
If you need a screenshot of them working in action, let me know I will post a Unigine benchmark using the two for you to see its actually working.