Actually, 3-way SLI doesn't seem that ridiculous if you already have 2 GTX/Ultra's. I'm actually considering it after seeing reviews of how well it scales with high IQ. I've been enjoying my GTXs for well over a year now (bought at a discount through EVGA Step-Up) and don't mind spending the extra dough. The only drawback I see (other than spending another $500 on a card) is I'll need a 1200W PSU - I can, however, re-use this in my next build.
Of course, you don't NEED to pay $1800 to play a game. As with anything, you do need to spend an inflated price if you want the fastest/best - it's the Mustang vs. Ferrari model. I think even nV understands not a lot of people are willing to spend this much for 25% more performance - 3-Way SLI, like Quad SLI, is more of an oooh-aaah product that will appeal to a small minority (contrarily, Quad SLI sucked compared to 3-Way SLI).
It's especially attractive to enthusiasts seeing that G100 probably won't be out until late spring or early summer (thanks to little competition from AMD in the high-end
).