It worked for me about 12 hours ago. Here are more detailed instructions, compiled from a combination of stuff I've read here and on other boards, as well as my own experience last night:
1. Download both install programs (8.0 and 8.1) from here:
http://windows.microsoft.com/en-US/windows-8/upgrade-product-key-only
Note that these are both very small downloads, about 5MB each.
2. Start the Win8 install program. It will ask you for a valid key, which you got from buying either a new Win8 CD retail, or from buying the super discount upgrade Win8 CD they offered last year. The new key you got if you upgraded to Media Center won't work, nor will keys from OEM Win8, nor will the generic keys that are posted for volume licensing CDs.
3. If it accepts the key, it will tell you that the key checks out, then it will mess around getting ready to download. As soon as it gives you an estimated time for the download to complete, kill it. There's no stop button, so just click on the red X box in the upper right corner.
4. Now start the Win 8.1 install. It will fail with an error message. Click on Finish.
5. Start the Win 8.1 install again. This time it will run. It won't ask for a key, but it will mess around and then start downloading with an estimated time to completion, just like the 8.0 program did. Only don't cancel this one

.
6. It doesn't ask you where to put it until after it finishes the download to god knows where (I assume some temp file on your C drive), when it asks whether you want to make a USB stick or ISO. You can then specify where to save the ISO image, for burning later.
Also, it doesn't ask you what version (pro or core) or what hardware (32 or 64 bit) you want. It decides what you get, apparently based on your current OS and the key you entered.
7. It evidently looks at your machine and compiles the ISO according to what it finds, so there is no use comparing hashes or even file size with other people, they are all different; I've seen forums where people report as low as 2.8 GB and as high as 3.8. Mine was 3.2 GB for 64-bit pro. However, after the download was complete, but before it asked me where I wanted the ISO, it did a file verification, so hopefully if it passed that, you are good to go.
I haven't tried it yet, but the consensus on the boards is that in order to do a clean install of 8.1 from the DVD you just burned, you need to use the proper generic key from the volume distribution page on technet,
http://technet.microsoft.com/en-us/library/jj612867.aspx
Note that the VL key works ONLY to get the install going; you need a valid Win8.0 key to validate the installation after it's done.
I've also read that you can get almost the same thing as a clean install by using the DVD to run a "reset" on your current Win 8 drive, as noted here:
http://windows.microsoft.com/en-us/windows-8/restore-refresh-reset-pc
Hope this helps.