It’s not a bad idea, but really needing code formatting is a bit rare – do we have it in the advanced tutorial? I think that’s where it would belong.
I also like @pfaffman’s suggestion of editing the default placeholder text quite a bit.
In the end, you may have to do what we eventually did at Stack Overflow and try to detect code that has been pasted in unformatted, and reject the reply if you see too much “code-like” text in the post – there’s discussion around a plugin for that here on meta.