PLEASE NOTE: Not all machines are supported by Windows 8.1 Preview at this time. Via Paul Thurrott.
I started an update of the office demo Surface Pro to Windows 8.1 Preview using the online update process. I started by browsing to http://preview.microsoft.com, and clicking Get It Now.
Note: you can also download the ISOs (x86 and x64) from the site (tomorrow) and from MSDN and TechNet.
This downloads and runs an update installer. This is a small update to Windows. In the case of the Surface Pro, it spent ages at “Configuring Windows features 100% …” after the reboot.
I then logged into Windows 8 again. A banner appeared, offering me the Windows 8.1 Preview. I clicked on Go To The Store in the banner.
Windows 8.1 Pro Preview was there for me to install. Nice. I clicked Download to get the 2.44GB installer. I forgot to change the power plan of the Surface Pro and it went to sleep. I went back in, changed the plan, and the download picked right up after a few moments.
After about 1 hour 20 minutes, the progress was at around 40% on my 8 Mbps ADSL (considered fast in Ireland 
).
10 minutes later it jumped to around 66%. This acceleration might have been due to the Build keynote stream that was aborted by Microsoft that I had been watching.
And then after 1 hour and 40 minutes I was prompted to reboot the Surface Pro. Oooh exciting! It then rebooted into a Setting Up <XX>% screen. Less than 3 minutes later that was done and a another reboot.
Now the Surface was Getting Devices Ready. That was quick, just 30 or so seconds. Then Getting Ready appeared, and then Applying PC Settings. That flew up to 100%. And another reboot.
It then was Setting Up A Few More Things with another % count. We were done within a minute. Then it was Getting Ready. Ooof! Are we there yet? Are we there yet? Are we there yet?
Wait …. License terms 
I wonder what happens if you don’t accept the license terms! Now it wants my password and dual factor phone number. A security code is instantly texted to me and I verify the machine.
Next up: Skydrive. I choose to use Skydrive. There’s nothing more than that apparently.
Now it’s the “Hi, we’re setting up things for you”. Damn you! You’ve already set up lots of things for me 
It’s been 10 minutes since the machine “finished” installing. A minute later and the Start Screen appears with my previously installed apps. And we’re done!
Edit1:
We’re not done. There’s up to 71 MB of updates to install. Now we’re done.