No. Windows 8 will not be an iPhone killer, nor an iPad killer. But it's the first step on a huge improvement. Perhaps Windows 9 will be the killer.
One of the problems that Windows 8 has is that it has no central focus on Music, Video, and Books/Mags, unlike the iPhone. There's Windows Media Player, but it's not as integrated as iTunes is. And it's not as central to the functioning of the Windows Phone. Ideally, a Windows Phone 8 or Tablet 8 doesn't need a central manager, but we all know people want to backup their device to someplace other than the cloud.
It's a huge first step, but it needs a lot more fine tuning before it can compete with the iPad. And certainly, prices will have to come down on tablets.