Demonstração prática do método .live() do jQuery e trabalhando com arrays no php

<?php
	if( $_SERVER['REQUEST_METHOD']=='POST' )
	{
		$values = Array();
		foreach( $_POST['conta'] AS $conta )
		{
			if( !empty( $conta ) )
				$values[] = "(NULL, '{$conta}')";
		}
		$sql = "INSERT INTO `table` ( `id`, `conta` ) VALUES ".implode( ', ', $values );
		echo $sql;
	}
?>
<html>
	<head>
		<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.5.1/jquery.min.js"></script>
		<script type="text/javascript">
			$(document).ready(function(){
				var i = 1;
				$('#campos input').live('blur', function(){
					if( $( this ).val()!='' && $('#campos input:last').val()!='' )
					{
					if( i<10 )//limitar em 10 campos
					{
						i++
						$('#campos').append( '<label>Conta '+i+':<input type="text" name="conta[]" value="" id="'+i+'" /></label>' )
							.find('input:last').focus();
						}
					}
				});
			});
		</script>
	</head>
	<body>
		<form action="" method="post">
			<fieldset id="campos">
				<label>Conta 1:<input type="text" name="conta[]" /></label>

			</fieldset><!-- /campos -->
			<label><input type="submit" name="ok" value="ok" /></label>
		</form>
	</body>
</html>