ellas,
here are a couple of the studies on oral yohimbine use...  these studies show yohimbine to be relatively effective orally..  
Am J Physiol  1996 Jan;270(1 Pt 1):E72-8 Visceral adiposity, increased adipocyte lipolysis, and metabolic dysfunction in obese postmenopausal women.Nicklas BJ, Rogus EM, Colman EG, Goldberg AP.
Int J Obes  1991 May;15(5):305-15 Plasma catecholamine levels and lipid mobilization induced by yohimbine in obese and non-obese women. Berlan M, Galitzky J, Riviere D, Foureau M, Tran MA, Flores R, Louvet JP, Houin G, Lafontan M.
Isr J Med Sci  1991 Oct;27(10):550-6  Does yohimbine act as a slimming drug?
Kucio C, Jonderko K, Piskorska D.
Eur J Clin Invest  1988 Dec;18(6):587-94 Alpha 2-antagonist compounds and lipid mobilization: evidence for a lipid mobilizing effect of oral yohimbine in healthy male volunteers. Galitzky J, Taouis M, Berlan M, Riviere D, Garrigues M, Lafontan M.
 there are also numerous other studies on isolated cells (which may be more relevant with limited systemic uptake due to local delivery) which show it to be considerably more effective, however these levels cannot be obtained with oral use due to drop-out of subjects.