In short, yes, green tea does cause an increase in metabolic rate. In a 2000 study from the department of physiology at the University of Geneva, researchers found that yes indeed, green tea extract, rich in catechin polyphenols is effective in increasing the body’s energy expenditure (and fat oxidation) for a 24 hour period.
The research involved subjects ingesting green tea at breakfast, lunch and dinner. Relative to the placebo, the green tea resulted in a thermogenic effect that promoted fat oxidation and a 24 hour increase in calorie expenditure.
Interestingly enough, the study also suggested that it wasn’t the caffeine that caused the effect. Treatment with equal amounts of strict caffeine in amounts equivalent to those found in the green tea had no noticable effect on metabolic rate.