Don't auto-append!

First off, VERY COOL module! I've been thinking that Drupal should have a Gravatar module and I was excited when I saw yours.

I see that you had $comment->picture in there at one point, but theme_user_picture() which gets called by phptemplate_comment() doesn't allow external images (a Drupal core bug, IMHO).

How about you add that as $comment->gravatar instead? Then themers could actually have some control over where the Gravatar image goes in the page?

$comment->gravatar = theme('gravatar', $grav_url, $comment->name, $comment->homepage);

And then you should have a module setting to choose:

(x) prepend the Gravatar image to the comment
( ) leave it up to the theme to add

if (variable_get('gravatar_prepend', TRUE)) {
  $comment->comment = $comment->gravatar . $comment->comment;
}

...just my 2 cents...