We discovered an article in Taiwanese magazine PC Home Advance that is sure to get NVIDIA's knickers in somewhat of a knot.
The article explains how to enable SLI support on a motherboard without official NVIDIA SLI certification.
If you recall back a little, NVIDIA made a policy that if motherboard makers wanted to add SLI support to their Intel X58 chipset based motherboards, they would need to pay for certification to enable support in NVIDIA's ForceWare drivers.
PC Home Advance in Taiwan managed to work out if you flash GIGABYTE's EX58-UD4 motherboard with the latest F6 GIGABYTE UD4P BIOS (this board has official SLI support) from the GIGABYTE website, it will enable SLI support on the cheaper UD4 model.
Since you are flashing your board with a different model BIOS, you cannot just use the BIOS QFlash utility or the Windows based @BIOS. Instead, you need to flash your UD4 board with a DOS BIOS flashing program called SPIFLASH - a quick Google reveals it is readily available online. Once you've downloaded the utility, you need to enter DOS mode with a boot disk and simply type in the command "SPIFLASH EX58UD4P.F6", then the BIOS flash update will proceed. Once you've rebooted and gone back into Windows, you'll have SLI support.
Remember that flashing your board with a different BIOS will more than likely void your warranty and do take care if you wish to try it out. Leave your results and comments over at our GIGABYTE tech support forum.