Any soap should work fine on clothes. Soaps work by having one end bond to oils and the other bond to water molecules if I remember correctly and since urushiol oil (the thing in poison ivy/oak that makes us break out) is an oil they'll stick to it and remove it from the clothes, just like grease on a pan. Its also supposed to work on any areas you came into contact with the ivy/oak if you wash with it before it manages to bond with your skin. Most sources say half an hour but I've heard anywhere from seconds to hours. Can't necessarily vouch for it but I've never gotten a bad outbreak by washing immediately. Another interesting thing to note is that rubbing alcohol is said to work for a few hours after, the downside is that it completely strips your skin of protection from future exposure for a day or so (so don't touch those clothes) making any contact much worse.

http://poisonivy.aesir.com/ had pretty accurate information by from what I can remember. Hey its been a few months since I needed a cure.