At last Microsoft have released a free install disk for Win 8.1 which will be great for folk who have no disk (although you will still need a key of course.)
Out of curiosity I tried this install media on one of my PCs with a windows 8 key. I always thought it was dumb you couldn't go straight to windows 8.1 but this fixes that issue.
One bad thing about this though is that it doesn't look like you can use this to do the "refresh" feature in windows 8. Every time I tried to use it, it would crash during the first part and go back to my desktop and say no changes were made. So its worth keeping your old DVD/USB media if you need to do a refresh.