I fixed my blog comments so that leading whitespace is now preserved — making it much more friendly to posted Python code.
It was a matter of adding these lines of PHP:
$comment_text = preg_replace("/^(\s*)/me", 'spaces_to_nbsp("$1")', $comment_text);
...where the implementation of spaces_to_nbsp() should be obvious. Old comments won't be fixed, but new ones can contain Python code without worrying about whitespace issues. At work we use Sharepoint, and I noticed that it handles leading whitespace the same way as the above — it would be nice to see more web apps doing this.