That is of course if the world doesn’t end. Jokes apart, the reason I think that the second half of 2012 is the right time for Windows 8 tablets is that it’ll guarantee the right mix of hardware will be available around that time. Quad core & higher ARM tablets would be the norm then with pretty powerful graphics (which would be a must if display resolutions hit the 2Kx1K levels with the iPad 3). Intel would also be closer to providing a compelling x86 based SoC in the tablet space. In fact, the tablet hardware is going to continue to scale up in leaps & bounds over the next 2-3 years before we reach a level of acceptable performance (just look at the roadmaps of the major SoC makers).
Apart from the hardware angle, the software landscape and usage model on tablets is also evolving. At the moment people are trying to mostly replicate the desktop or smartphone UI paradigms on the tablet. For Windows 8, it all boils down to how well Microsoft is able to adapt their MS Office UI to the tablet. Then again, we could also be looking at tablets being used as a laptop\desktop replacement when docked – Apple seems to be heading that way with their Thunderbolt display (MacBooks for now).
So, don’t fret over the timelines, and instead be excited over the emerging paradigms over the next couple of years.
P.S. Extremetech has a nice how-to for building your own Windows 8 tablet (you will need an existing Windows 7 tablet of course)