How does a tablet/smartphone cpu compare to desktop cpu?

Tablet/smartphone cpu are becoming more powerful, they are now reaching 1-2 ghz and have 2-4 cores but how do they really compare to desktop cpu?
I know that they may be a lot less powerfull but how much, if you compare a core 2 duo to the latest mobile arm cpu.

You really can't compare them, you can't judge by speed, nor instructions per second, nor how fast they open up applications because the applications are coded differently. To compare them you'd need to run the same benchmark on both devices which you can't do.

It's enough to say that they are quite a bit slower. I'd estimate the fastest phone is not even 1/4 the speed of a dual core CPU.