Here
http://www.votehemp.com/PDF/Transdermal_THC_final.PDF is a fairly pertinent article (not an abstract). The permeability of the skin to THC if poor, but can be enhanced using traditional carrier molecules, like propylene glycol. In fact, carbon tetrachloride
http://www.patentstorm.us/patents/6113940/description.html is used as a carrier in a THC patch developed for transdermal application. So while it's very unlikely you could absorb enough THC through your skin from just handling plants,
the very solvents you use to clean your bare fingers (isopropyl alcohol, vegetable oil, etc.) will actually increase the permeability of your skin to THC. So while the levels you absorb will still be very low, it's probably wise to do as the doc says, and use gloves if you're worried about testing.
Thanks for asking the question, it's one I've often wondered about as I was scraping the resin off my fingers.