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If the contents of the iframe are on a different domain than the 'wrapper' site, then there's nothing you can do.
If you had access to the form code you could change the target to '_top'. That way the form's actal site shouldn't be affected, but when it's submitted it should break out of your frame.
[edited by: Dabrowski at 5:46 pm (utc) on Dec. 21, 2007]