Assuming they will be using multiple operating systems and/or different browsers, you'll have to take into account that they could enter different characters for a new line. Many UNIX variants use just \n, while Windows machines use \r\n, and Macintoshes use \r. Of course, depending on the program, these can change, however.
The function [man]nl2br/man changes all of these character combinations into a <br /> and a new line, which you can then [man]str_replace/man into whatever you like.
Hope that helps.
