[General Topics] sp3dev:
Actually it's an arueable question whether dual-core cpus are an overkill today, they have several advantages but most of those can be applied to netbooks and tablets rather than phones.

1. When there are several CPUs, multi-threaded applications can be really run concurrently (and basically, even if one application is performing, the scheduling overhead for multi-core system is lower and background tasks like gui/hardware drivers can be executed on a separate core).

2. Another use case (although this is a misuse and abuse of CPU anyway) is the use of multi-core systems for encoding/decoding media. It brings absolutely no advantages to the end user, but when the CPU is powerful enough to handle the media stream, one may use it instead of a proper DSP processor which Google will likely be doing for VP8/WebM

3. SMPs can be useful in tablets and netbooks - for example, tegra2 will outperform intel atom in most cases (first of all, it is dual-core. and secondly, it has a very powerful GPU). I am personally using debian on my tablet (in chroot though) and many people are using ubuntu on toshiba ac100 - arm SoCs are a fun to hack and give an incredible battery life. But this is IMHO only acceptable for geeks like us and I think dual-core (or x-whatever-core) ARM CPUs will be useful for consumers (hate this word but whatever) if some vendor releases a device which will run a full-fledged linux distro with LibreOffice, math packages like octave/maxima, development environments like kdevelop so that it can be used as an equal replacement of an x86 netbook.

As for the popular arguement about power consumption - surprisingly, but there is little correlation between the number of cores and power drain. Newer SoCs are more energy efficient because they have improvements in technical process (literally the length of wires inside the chip), more devices are integrated into one chip, more processing blocks can be put to sleep states. Even if you compare a qualcomm qsd8250 running at 1GHz with a GPU enabled, it will use less power than an old 520 MHz intel pxa270. Besides, as I have already mentioned, a multiprocessor system can execute tasks concurrently which means that the computation will take less time and the processor will spend more time in a power-saving state.

Basically multi-cores are a popular trend and is a good way to make consumers pay for new toys. For me personally the reasons to change a device have always been either the age of the device (when it literally began to fall apart) or the real improvements in hardware (I updated from Asus P525 to Xperia X1 because ever since I had my first pda I was frustrated by the tiny 32 or 64 mb ram and awful screens with large pixels that were really causing pain in eyes if one used them for long) but unfortunately the situation now is the same as it is in the desktop world - software quality is getting worse even faster than hardware improves. Hence we see crap like java and other managed code on PDAs and applications that require like 10 Mb ram to perform simple functions (which were like 100 Kb back in winmo days). I do admit that using more ram can allow to use more efficient algorithms (to reduce their computational complexity) and managed code allows for higher portability - but hey, we know that commercial software is not developed with the ideas of efficiency in mind - the only things corporations care about are writing the application as quick as possible and hide the source code.