If you're working on a site that posts personal information out there on the web - beware! There are any number of spambots and programs out there that work to search through pages and find anything that even vaugely looks like an email address and add it to it's arsenal. Thankfully, there are articles like this one from DevArticles that show you how to protect your users.

In Protect Your Users Email With PHP and MySQL, the author shows you how to create a system (similar to something eBay uses), that will hide the user's email address, but still allow someone to contact them. The email sends through a PHP script on the server-side, and never has to show the person sending the message the other person's email address.

It's a good idea to use something like this, especially if your site relies pretty heavily on user interaction. That's another reason why I make the email address in the comments on here optional. The less spam the better, right?