Not sure what the "Good Grief" is about. This post was originally put up in Feb 2009 several months before the Helping Families Act was signed into law. But it is certainly a good addition to this thread to mention Title VII of that act titled -- PROTECTING TENANTS AT FORECLOSURE ACT
You can see the full text of the act here: http://thomas.loc.gov/home/gpoxmlc111/s896_enr.xml
Or see the next post (Thanks AK, I updated the link)
Your description, however, is not quite accurate. An existing lease must be honored until its termination. If the termination date is less than 90 days from the date of notice of the completion of the foreclosure, then a minimum of 90 days notice to vacate must be given. The 90 days is not tacked onto the end of the lease. There is also an exception, as you noted, where the purchaser plans to occupy the residence. In that case, the lease can be terminated before its end with 90 days notice.
There is nothing in the act that allows tenants to stop paying rent during prior to vacating, either before foreclosure is final or afterwards. Upon notice of the finalization of the foreclosure, the tenant will then pay rent to the new owner.