Is using SLI or Crossfire a transparent process to the video game developers? Or do they have to consider SLI and Crossfire when they develop their game? Is it all the same DX calls regardless of the code path taken (SLI/Crossfire or not)?
I am not entirely sure if it is transparent to the devs...
Support comes from drivers... this is why you see 'game ready' drivers from NVIDIA and 'hotfixes'(for specific titles) for AMD. I am going to GUESS it is transparent or mostly transparent to the devs based on this point only... if it was transparent to the devs, you would think that it wouldn't take so long for SLI/CFx support to come out. They wait until release day or even later to put those drivers out. I highly doubt they code multi-gpu support that late in the game...
... that said, it is just a guess. I have no idea.
SLI support can be added at the driver level if the game lacks sli support. As eatherdog stated there are game ready drivers with sli support. There is also a tool that can mod the nvidia drivers to add sli support to a game that lacks it.
Lets break some layers down in order to answer this question.
Games live on the software level, and must communicate down to the firmware level of the GPU to instruct what graphics to display on the screen. We understand that instructions are sent back and forth, but what are these instructions? Each game is built off an engine that has libraries to interact with the drivers. The drives, than communicate down to the firmware, and thus the hardware itself. Each game must be built around a library of available commands that are first available at the firmware level, but trimmed as we move up to the game software level. Not all firmware instructions are available, because the developers will decide which are most important.
Given this break down, we can see that the SLI/XFIRE instructions would be available up to the game software, if allowed. This is what most people understand, developers must enable multi-gpu rendering, and they can use even more instructions to smooth out performance. However, a driver can recognize specific instructions and instruct the firmware to act differently, this is where driver optimization comes into play and why AMD and Nvidia make sure to stay on top of driver support. Game developers cannot always keep on top of the latest and greatest, because they are more worried about the game running on a variety of hardware. Driver support relieves some of the effort of the game developers, while also creating a customer support role (a reason as to why Nvidia is getting a higher market share atm).
Dolk's reply makes it seem to me that devs wouldn't really want to bother w/Crossfire-SLI at all. After all, if most of your market is consoles, why would you want to waste development dollars on a shrinking market?
ED quoted an article where the author said Crossfire/SLI support for console ports is becoming abysmal.