You would know by now (today 3-22-09), if you have contacted the substance and are allergic. There seems to be a period of time after the initial contact that you can wash it off, I believe there are products available for this purpose, but after a certain amount of time, it does not come off or kind of sets in, if you research it on the net you'll get some good information on it. I have read that urishiol is so potent, the amount/drop held on the head of a needle is enough to make the whole world itch, whether that is true or not, can't say, but there is no doubt as to how just a trace amount or limited contact with the substance can cause a very irritating rash if you are allergic to it.
Apparently, the effectiveness of remedies, like some that were mentioned here, differs among people, I used to get it more often back when I was a dozer operator, summer time, shorts on, take a step out of the work area to water the plants, when working in the woods, changed my habits a bit after this one job, and some of those remedies, gasoline, clorox bleach, did not seem to work for me though, after the rash, maybe it was meant to clean off the substance before it sets in.
That bout of it I got in in '04 was just unreal, I mean it woke me up about 4:00 a.m., and just tore up my forearms, for at least a week, itch itch itch....even with treatment, now I am very careful to find it first. I was working in it and did not know, trying to get metal "T" posts in along this old stone wall, rocks make that task difficult, the plant was not well established but there, so I got a darned good dose of it, and it scarred the heck out of me, temporarily, you can use the same therapy as a burn, Vitamin E, Cocoa Butter, or similar after it subsides. I also think the potency was at it's peak, I've never had a reaction to it like this before. The plants in this area have become more prolific, been trying to get it to grow up onto the fence, so the thieves on ATV's will have to deal with it if they try to take the fence down, I have broken the plant and carefully applied the substance, where one might touch it.
That substance if on work clothes, gloves, shoes etc., can cause secondary reactions, not as potent, but still quite an irritant. I had to do some fence repairs at our other place, stone wall nearby, hedgerow with hickory trees, though the plant is there, it does not seem to overtake or become rampant, just there mixed it, got a couple of small spots of it, but you could tell, it was potent and or I'm very allergic to it. Realizing I was working in it, I treated my gloves, clothing and shoes with respect, made sure not to touch them, this substance can stay potent for a long time, you can break the plant and get it in the winter. One thing to avoid is cutting into it with a string trimmer, mower or anything that will have the same effect of spreading the sap/juices from the plant.... that and burning it, both can do some serious damage to a person.
