A single space looks just as incorrect with a proportional font as it does with a fixed pitch one. The best historical fact in the web page linked above is the use of the wedge spacer on the old linotype machine. The typewriter versus computer explanation doesn't fly. Of you want to verify this, try a paragraph in a fixed pitch font and proportional with single and double spaces. I tend to always use two spaces as it was the way I was taught and it's now just force of habit. It doesn't hurt forum postings either way and it doesn't affect HTML.

The rivers of blank space within a line argument is also weak (from the link). This has less to do with two spaces after punctuation, and more to do with poor justification.

Bruno