I should have asked previously. Why do you want to open a new window? What is the use case? You are the first to ask for this so I just wanted to get a better understanding of what you are doing.
I am working on a site on a Ning platform. I have inserted the form on a sign up page and when I redirect to a offsite page it opens within the ning page. The site isn't public yet but the link is here if you want to see. Fill in form and submit an you will see (still in testing so I will just delete the entry :-)
I'm glad I asked. I understand the issue now. This is actually something I want to fix - when a user embeds a forms provide an option that makes the redirect take over the entire HTML parent page.