The downside: It will arrive in Mac flavors only for now. I can see the Mac community going nuts for this and touting it over the PC community for as long as they can. Will it push Mac sales over PC? Only time will tell.
Its a known fact that AMD is working on Dual Core on one chip designs as we speak and that they are to be publicly available in early 2005 as well. It will be interesting to see how Intel will counter a Dual Core Mac and AMD design if they are first to market.
Dual Core IBM Processor