Tough call IMO. If one must have the top system, sure it's likely worth it. OTOH, if someone goes single vidcard now then adds in a second card later as an upgrade, it might be worth it. Personally, if I went SLI it would be the second way, buy a Mid-level vidcard now, then buy another later when it became cheap.
It'll be at least six months before I upgrade from my current XP 2800+ and when I do I think I'll get a SLI mobo, but whether I get an Nvidia based vidcard or not is stillnin the air. I'm hoping that ATI's SLI setup will be close to market and that the Technical details of it will be available at that time to help me make a decision in that regard.