Apparently the plant poisoning in the movie (and postulated in the book) weren't correct. If you read the Wikipedia on McCandless[1] research into how he died has made it clear that it was just plain old rabbit starvation[2].

[1] http://en.wikipedia.org/wiki/Christopher_McCandless

[2] http://en.wikipedia.org/wiki/Rabbit_starvation

Finally watched that movie on Tuesday night, I loved it, a very interesting story - made we want to get outside more!