When devices began to ship with dual-core chips the market wasn't quite ready for them and anything but high-end games would take advantage of the added core. However now, dual-core is all but a requirement for a half-decent Android device, with prety much all apps now being optimised for multi-thread and therefore take advantage of the added power. Quad-core has been available since the end of last year but it's featured in few phones and is mostly present in tablets - something I believe to be due to heat-dissipation. However, quad-core is on its way to the market now in the form of Qualcomm's Snapdragon S4 Pro line of series.
MediaTek - a company that provides a lot of ARM based SoCs to low-cost manufacturers in China - have just finished preparations for their Quad-core SoCs and are said to be supplying them before 2012 ends. Whilst it's unlikely we'll see such a chip make it to devices anywhere other than China or Europe but it's certainly good to see other manufacturers perfect the process because it certainly suggests that quad-core processors are becoming the norm. Which can only be a good thing, the faster this sort of power becomes available to everyone then the fast we'll start to see an improvement on the applications that we use everyday.