jQuery extension to prevent line breaks in textareas. Demo: http://jsfiddle.net/nicz/Z77YQ/1/

jquery-nolinebreaks.js

/**
 * Disallow line breaks in textareas. Line breaks are converted to
 * a space character or any other replacement.
 * Usage examples:
 *   // Convert line breaks only once to spaces (default):
 *   $('textarea').nolinebreaks();
 * 
 *   // Convert line breaks only once to something else:
 *   $('textarea').nolinebreaks(' / ');
 * 
 *   // Convert line breaks once and on any change:
 *   $('textarea').bind('keyup change', function() { $(this).nolinebreaks(); }).trigger('change');
 */
(function($) {
	$.fn.nolinebreaks = function(replacement) {
		replacement = (typeof replacement === 'undefined') ? ' ' : replacement;

		var $textarea = this,
			value = $textarea.val(),
			end = value.substring(value.length - replacement.length - 1, value.length - 1),
			newValue = value.replace(/\n+/g, end === replacement ? '' : replacement);

		if (value !== newValue) {
			$textarea.val(newValue);
		}

		return this;
	};
}(jQuery));