Photoshop suffers from three main bottlenecks; CPU speed, memory and hard drive speed. Working on only one bottleneck won't be able to speed up Photoshop unless you also address the remaining bottlenecks at the same time.
Yes, Photoshop can use multi-CPU's has been since version 3.0. Problem is that when you double the processors, you don't get double the performance...you get maybe 1.5-1.6X for each additional processor and that is mitigated even more when processors have to share memory as current multiple dual cores or quad cores are involved. Will Photoshop take advantage of all 8 cores in the new Mac Pro? Yes...but it ain't gonna be 8x the speed.
The other thing is we are STILL memory limited by both OS's...Mac Leopard will help as did Vista 64 bit...but not enough yet to make Adobe feel compelled to do the enormous work for relatively little benefit. But even if an app can't take advantage of over 4 gigs...Photoshop DOES speed up considerably (both platforms) when you have 6 gigs or more...why? Cause the OS can use all the rest and that means less page-outs...every other CPU task besides Photoshop uses and needs ram...so more ram is still better even if Photoshop can't access it directly.
Then you get to hard drive speed...the current dual quad core can have 4 SATA II drives...and each SATA port is its own port so you can array 3 drives to be stripped (it's unclear yet if you can boot off an arrayed partition-X-serve can, the G5's couldn't so I don't know if you could do a 4 drive stripped array). Up to a point past 4 drives you run into the same general issues you run into with multiple CPUs...the speed increase falls off.
But, get one of the new dual/dual or dual/quad cores, put in 8 gigs of ram and array at least 2 drives for scratch and you will have a smoking' fast Photoshop machine...Will they get faster? Yes...but a dual/quad core with enough ram and fast drives will run as fast as you can, at the moment.