Actually, I agree with isuppli, that a lot of current Wii owners will get a second console. Most current Wii owners are gamers (previous PS2/GC owners) and will want some of the games not available on Wii. However, PS3 being the only one to benefit from this and not 360 seems like wishful thinking.
Wii is also gaining a lot of new & lasped gamers, and these will NOT buy a second console in any sort of significant numbers. Also new gamers are only at the tip of the iceberg for Wii. The majority will buy a Wii once the price drops to $199 or lower and becomes readily available in stores.
Also a lot of 360/PS3 owners will also get a Wii for some of the unique games only available on Wii.
It seems isuppli looks only at reasons why PS3 should sell, and not at reasons why Wii or 360 should sell.
I suppose PSP will outsell DS according to isuppli too.