Here's some quick stats on Poison Ivy. 1 out of every 4 persons are allergic to urishiol, the oil that causes the rash. Of the 3 in 4 who are not allergic, there is a 95% chance that they will develop the allergy at some point in their life. So... the rule: Treat poison ivy like you're allergic to it.
I saw someone already posted the info about tecnu. (I put that in a previous tread a couple weeks ago). Tecnu makes 2 products: a cleanser and a barrier. The cleanser actually removes the oil and prevents you from getting the rash. you can apply it up to 8 hours after coming in contact with poison ivy/oak/sumac. It works great and i SWEAR by the stuff.
The barrier prevents oil from getting to the skin. You put it on before you go out, and wash it off when you return. heres a link for the product so you know what it looks like. you can get it at any local pharmacy.
http://www.benmeadows.com/search/eproduct.asp?R=130220&dept_id=11825&pf_id=1452
steve