Pete's right. The inflammatory agent of poison ivy/oak/sumac is Urushiol. It's an oil so water and soap won't do much more than spread it; not good. Do a search for tecnu. It removes the urushiol. No connection.
I've always read that washing thoroughly with soap and water ASAP is the best reaction to contact.
Center for Diseases Control says:
"Immediately rinse skin with rubbing alcohol, specialized poison plant washes, degreasing soap (such as dishwashing soap) or detergent, and lots of water.
Rinse frequently so that wash solutions do not dry on the skin and further spread the urushiol."
Of course the real problem is when you don't know you've contacted poison ivy (or similar) ... until the rash start showing up. By then its typically too late.
Besides the more commonly known poisonous plants, don't forget the nightmare of wild parsnip
. Its leaves have a "solar accelerator" that can lead to 2nd degree burns and will pigment the skin for years.