The only problem with the target attribute is that is is not valid in xhtml. An alternate way to do it is to replace the control with this markup:
<a href='http://www.mojoportal.com' onclick="window.open(this.href,'_blank');return false;">Powered by mojoPortal</a>
and of course you are also free to just remove it completely though we do appreciate it when people keep the link.
Best,
Joe