- The logo is actually not a trademark, but the word "Moodle" certainly is. I have no problems with people using the word or the logo as long as it does in fact refer to the product Moodle (or Moodle-related services).
- There is no license requirement to keep the Moodle logo or link on the front page. I could have put a copyright notice in there (which would not have been removable under the GPL) but chose not to, because I don't like it when other projects do that.
- Acknowledge they are using Moodle
- Provide a link to moodle.org
I've joked before that I won't help people who don't show a Moodle link because I can never tell what software package they are using.
Personally I don't care how you do it. Many sites use a text link, which is fine, and others do modify the colours or the size, which is also fine. I am keenly aware and understanding of the need to make your site look as good as possible (this in itself is a great advert for Moodle! )