personally, I'm not sure if I'm gonna get an SLI mobo or not. I certainly wouldn't be opposed to it, but I'm not going to go out of my way for it. If the Asus SLI board is the first to market, and doesn't cost over $200, then I might go for it; otherwise, I'll probably get the Gigabyte Ultra board.
SLI isn't necessarily a bad deal if you use it for an upgrade. For example, say you get a 6800GT for $400. Then, whenever the next big (not refresh, but core change) upgrade in graphics occurs, a card in the same range as the 6800GT (say 7800GT or whatever) comes out, and is $400-500 (the next generation will probably run $450 MSRP, if ATI's pricing for its X850 line is any indication, since they are moving price points to $350, $450, and $550 for top end range cards). You could shell out for this card, spending another $450, or you could throw in another 6800GT for $200 or so (like the 9800 Pro is now). This would yield you around a 70-80% increase in performance for much lower cost. In most GPU limited cases, the gain from another card is MUCH higher than 50%, by the way. Anyways, the point is that SLI does present some nice upgrade potential. As a straight out of the box solution, though, you are right, it is not currently a good value for the costs involved, unless you are disgustingly rich. Also, SLI is not exactly going away; ATI has plans to release its own version with the R520 (along with PS3.0, BTW), and will also release a version of their Xpress 200 chipset featuring dual graphics support.
One last thing - I really don't think it's accurate to say that an X800XT "owns" a 6800 Ultra. Being maybe 2% faster does not constitute "ownage" especially when it's lacking in its feature set. "Ownage" is the X700XT and 6600GT's performance relative to a 9800 Pro. Personally, I think it's pretty much a wash between the X800XT and the 6800Ultra, not that I'm too concerned about it, since I'll be looking in the $200 range for my next card.