I'm surprised the percentages have dropped so little. I'm also surprised these analysts never mentioned that given the power of PCs over the past few years they don't need to be replaced for the next faster thing. I believe that is the bigger issue along with the move to tablets for daily content consumption, it's a lot more efficient to have instant on tablet to get your daily socialization in through out the day then to boot up a pc. Windows 8 won't change anything, the OS brings new life to older hardware and it's also going to push more tablet and hybrid sales. Rather than looking ONLY at the PC as dropping in sales they should put PCs, Tablets and hybrids together into the same group and look at it as computing devices.
As for the enterprise it has little to do with Windows 8 also, enterprises don't jump on the next OS right away. Also again there is no need to replace hardware that has more performance than required even today on hardware that is a couple of years old. Add to that the move to VDI as well which also takes a chunk out of PC sales at the enterprise. Whatever OEMs will lose in PC sales they will make up in tablets or hybrids while still attracting those that NEED power house PCs like gamers etc.