SLI is to sell more graphic cards.

Dual GPU is to sell more GPU ...... since they can't make enough nowadays X800 and 6800 series for whatever reasons (not that they don't want your money) it becomes just a conversation piece.

Advantage of SLI
1) Create more mother boards sales for the "needy" despite twice the money.
2) Double the sales of graphic cards; hopefully.
3) Pushing game developer for more.
Disadvange of SLI
1) Small market but it can just be as small as water cooling used to be.
2) Motherboard makers stand a better chance to lose; not the graphic card makers.

Dual GPU
Advantage Dual GPU
1) No need for motherboard upgrade.
2) Make good use of "second graded GPU" - putting two R350 together might beat two cuts higher card, i.e. continue production from tooling already paid for.

Disadvange Dual GPU
1) Can't charge twice the money.

Can you think of any??;)

Well, dual GPU setups cost less then two identical SLI cards. Besides, the SLI cards need to be identical, in fact, forcing you a bit to not wait too long before buying the other graphics card.

Economically, Dual GPU is thus better for all parties. Btw, for overclockers: less different cards and components, like on Dual GPUs mean more stable overclock...