I completely agree that you shouldn't open content in new windows/tabs and always try and advise this to clients. One area where they seem to take issue with this is on forms where the last item is an 'agree to the terms and conditions' checkbox and the 'terms and conditions' text is a link to that page.
This is where often clients will insist on the page opening in a new window/tab or sometimes a lightbox type window (is this any better?). The reason given is that the user has completed the form and if they leave the page, they may lose all the form fields they have completed.
Just wondering what peoples views are on this and what people usually do in this circumstance.