Naturally. But desktop CPUs are generally faster per clock. Also factor in that the price of a mobile CPU is higher and for the same price you can get a faster desktop chip.
For example: The t7700 (2.4Ghz) $335
http://www.newegg.com/Product/Product.aspx?Item=N82E16819111316
You can get a 3.0Ghz Conroe for much less, $280. You can even get a Quad 2.66Ghz under $300. So while the comparison at the same speed is understood, it wouldnt make much sense. And even if you went that route (a similar speed desktop chip) it would work out to be much cheaper while offering less performance.
Plus, you can go with faster RAM as well, further improving speed. Lower latency RAM, etc etc.
That is where the "destroying" comes in. At the prices of the iMac one can build a machine that will destroy it in performance.
Now, going back to the Mac Pro. Many of you think it is a good value at $2500. However, you can do a single quad 2.66Ghz Dell T7400 with similar specs within the same price range ~$2624. That is including the Harpertown CPU, which clock for clock will perform better than the 2.66Ghz Woodcrest. And also includes a 3yr on site warranty standard
Now if you look at the Apple octo-core, you are looking a $4000, everything else stock. For the Dell T7400 I configured it with the 2.83Ghz quad core chips (E5440) as at that clock, the 45nm, and the added cache it would easily perform on par with the 3.0Ghz Octo Mac Pro. I upped it to a standard Mac Pro specs and it sits at $4263. Mind you that includes a 3yr next day on site warranty.
For someone looking at the two machines it is a no brainer unless you are tied to OS X. You get new tech and a 3yr warranty. Once you add AppleCare to the Mac Pro there is virtually no difference. Same price and new tech, how can anyone who isn't tied to Apple argue with that?
This of course isn't to say the Mac Pro isn't better. I will always buy Apple as I love OS X and I love their machines. However, try to see the other side and why it would be compelling if OS X isn't involved.
I of course don't think Macs are the whole are twice the price. I also don't think they command some kind of premium on the whole. In the case of the Mac Pro, they could be more competitive as they made themselves out to be when they released. I have no doubt that they will do it soon. But as of now, the Mac Pro doesn't offer the value for your dollar IMO.