SLI is an abysmal upgrade path.
Why is that?
According to the specs, the Titan is 85% of the 690 which is basically GTX 680 in SLI. So someone that has a 680 SLI or 690 will get better performance than the Titan (unless it turns out the Titan is superior for other reasons).
I guess my point is, I imagine a lot of people will get the Titan to SLI them. Especially if you are gaming at a higher resolution, some of these games need SLI to perform at max. At least, that has been my experience. Going from a single 680 to SLI 680 has been a big performance bump - and even some of the most demanding games are pushing both cards to their limit to achieve it. So if someone is going to get a Titan at these higher resolutions, I can't imagine they are going to get the best performance possible with just one.