HTML5 brought the autofocus attribute for input fields. I suggest using it as much as possible since it is a simple and useful feature supported by all major browsers (and simply ignored by others).
Use it on any 'new X' form in the first input field (like the subject of a new thread).
Even more useful, use it on input fields that got rejected after posting (like invalid password) so you can start typing the correct input right away.
It won't hurt those who still feel better when they click the respective field. And it is definitely easy to implement.
edit: also this.