http://www.aad.org/pamphlets/PoisonIvy.html
Great article presented by American Academy of Dermatology

The Poison Ivy, Oak and Sumac Information Center
This is the ULTIMATE site for information on the topic.

other references used:
http://www.albion.edu/fac/biol/skean/toxirapg.htm
http://www.albion.edu/fac/biol/skean/toxivepg.htm
http://www.floridaplants.com/Eflora/poisonivy.htm
http://www.rook.org/earl/bwca/nature/shrubs/toxicodendron.html
http://www.biosurvey.ou.edu/okwild/poisonivy.html
http://www.sbceo.k12.ca.us/~mcssb/sbpanda/poison_oak.html
http://www.medicinenet.com/Script/Main/Art.asp?li=MNI&d=46&f=451&ArticleKey=451
http://www.fda.gov/fdac/features/796_ivy.html
http://www.io.com/~jparra/poisonous.html    Awesome resource!!!!