Multi-core processors on smart phones are inevitable I think (for the same reason that we're there already on desktop hardware) but the time hasn't come just yet.

Smart phone processors are currently pushing past the 1Ghz region and if you recall, Moore's law of processor clock speed increase on desktop hardware hit a sudden and abrupt wall at around 3Ghz back around 2003 or so - due to fundamental limitations in transistor technology. (Essentially, making transistors that can operate faster than 3Ghz is very difficult and specialized, and can't be done in bulk on traditional silicon wafers used for CPU's)

Instead, maximum clock speeds have remained at around 3Ghz for the last 7 years, and other techniques were sought to increase performance per clock cycle - like deeper pipe-lining, more advanced branch prediction, much larger L1 and L2 caches, putting the memory controller on the CPU die to reduce memory access latency, and so on.

The problem with a lot of these techniques from a mobile perspective is they introduce a HUGE amount of complexity in the chips, which means lots of transistors, lots of heat and power use, and simply don't lend themselves well to mobile processors which need to use a bare minimum of power, and be very small and simple, to allow room on the die for the other SoC components like the graphics processor.

We're now starting to reach the limits of these techniques on desktop processors, and the only way forward is multiple cores, and re-architecting both the operating systems and applications to scale well across multiple cores.

From a mobile processor perspective once we start nearing the per-core frequency limitations I think it will be more power and complexity efficient to go with multiple cores, than it would be to create a single very complex core that gets a lot more performance per clock cycle, the route desktop processors took.

Another advantage of multiple core processors in a mobile device is that it should be possible to switch off idle cores, (I believe multi-core ARM architecture already supports this) allowing the OS to run on only a single core when doing low power operations like background mail checking while the phone is locked - with a significantly reduced power consumption compared to running all the cores all the time.

Give it another 2 or 3 years and I think multi-core mobile processors will be fairly common - hopefully Apple and Google are working to ensure that the OS and development tools will work efficiently with multiple cores.