I have 3 more suggestions:
1) make it easy for everyone who participated (ie, sent in their best guess at answers) to get the official answers. If the official method of turning in our efforts is via e-mail, then at least send an e-mail back to the participants with a link to the answers when they are posted.
2) clearly state in the article up-front what date the answers will be available and then honor that date.
3) do not post the answers in the article itself. That way, people in the future can still play. Instead, when the answers are available, put a link to the answers on the puzzle/article page. That way, when people are ready to check their answers, they can.