NightGhost wrote:Just so there's no confusion, you're correct that 1/8" = 3.175mm, but those jacks are usually referred to as 3.5mm (the actual measurement is somewhere in between).
As far a what is standard, that depends on the device (e.g. 2.5mm is more common on cell phones, as you stated), and there has already been discussion about adapters for devices other than the Xbox controller.
Well, speaking (unfortunately) as an American, every Frys, RadioShack, and Best Buy has these listed as 1/8", especially for adapters that go from 1/4" to 1/8" as the metric system is rarely used (unless you're a serious scientist or engineer.) The 2.5mm TRS standard was just recently introduced (relatively speaking) so it is still not quite as widespread, and the main standard currently remains 3.175mm for most TRS jack devices or 6.35mm for almost every audio application/device bigger than a laptop.