The new Nexus 5 processor (2.3 ghz quad-core) is about 11,000 Dhrystone MIPS, roughly equal to the best dual-core processors of 2005/2006.
This suggests that at this moment in time smartphone processors are roughly 7 years behind general purpose CPUs.
Then I remembered that in 1995, I got an HP 200LX, a handheld PDA. It had an 8mhz 8086-equivalent processor, which was roughly equal to what I had in 1988 when I had an IBM PC clone at 8mhz. So then too, they were 7 years behind desktop processors.
It looks like it’s a fairly constant ratio, over nearly a twenty year period.