All combinations of 6 letters .com are taken. All 16k words of the dictionary are taken in English. It wasn't even the case 2 years ago. I wonder whether pg's impression that if is possible to get a .com is outdated.
Do you mean this literally for any 6 random characters? It rings true for 4, but I believe you can easily find a 5 character domain name even now. In fact, I just registered three five character and a six character domains without much trouble.