Wouldn't that penalize people who write comments in their editor, then copy-paste into the comment box?

Wow. Who does that? I do not mean this in a sarcastic manner, but this is the first time I hear of it.

Many people. There's even a moderately popular firefox plugin to automate the process (the name escapes me) - it invokes your editor for you, then when you save and exit it copies the temp file into the textarea.

I'm not so sure. I think I could fake it.

Bayesian analysis on not just the content, but the way the content was written. It'd be fakable, but not easy.