There's a lot of misinformation here about how SLI works and under what conditions.
Prior to Nehalem and X58, SLI would only work on nvidia chipset based motherboards. However, this had nothing to do with the motherboard or chipset, it was actually a check performed by the drivers... if a suitable nvidia chipset was present, SLI would be enabled, otherwise it wouldn't. Someone actually hacked nvidia drivers to work on non nvidia motherboards a couple of years ago but nvidia quickly rectified that and encrypted the keys to ensure that didn't happen again.
On Nehalem and X58, nvidia (so far) cannot develop a chipset replacement for X58 (tylersburg) so rather than be locked out of the multi-gpu market on this new platform, decided to offer motherboard manufacturers two options to get SLI support working on their boards:
1. Initially Nvidia said board vendors could implement the NF200 PCIe bridge chip which would not only enable SLI, but also offer a 2-1 PCIe bridge to provide additional PCIe lanes on the motherboard. The problem with this solution was that the chip runs hot and takes precious board real-estate so many motherboard manufacturers complained, leading to...
2. Under pressure from top-tier board manufacturers like ASUS, Gigabyte, and others, Nvidia offered to license SLI on a fee per board basis and by doing so, would enable the manufacturers to enable SLI flags in BIOS. Thus offering a completely hardware free solution for SLI.
Nvidia charges around $30-$50 for either approach on a per mainboard basis.
Note that it's up to the drivers to detect either the presence of the NF200 or the licensed BIOS flags implemented by the motherboard manufacturer.
Hence, it's not true that X58/Tylersburg supports SLI... the chipset really has nothing to do with whether SLI is supported or not. It's all about whether the vendor implemented either of the two solutions outlined above.
Finally, SLI benefits from, but does not require the SLI bridge connectors. The SLI bridge connectors that connect the cards at the top enable direct chip to chip communication without using the PCIe bus which can provide a performance improvement, especially at high resolutions. However SLI will run without bridge connector and use the PCIe bus instead if it has to although stuttering is a common problem when not using the bridge.
As for Crossfire, ATI doesn't require motherboard manufacturers to license anything. ATI drivers simply look for the presence of multiple compatible cards in the system, and if present, enable Crossfire. Hence, Crossfire works on any system.
With respect to multi-gpu support on the Mac Pro, unless Apple implements either of the two solutions above, SLI will not work on the Mac Pro. Apple has no need or interest in doing this for obvious reasons (i.e. supporting multi-gpu scaling under windows just isn't a priority for Apple). Since Crossfire works on any Windows system, it will obviously work on the Mac Pro under Windows.
Hopefully that clarifies things a bit.
