The DIS shirts and buttons make it the easiest to identify DISers unmistakably. I don't really like the shirts on me, but I've been asked countless times about my DIS button - sometimes by CM's, sometimes by DISers, and sometimes just by strangers who want to know what it is.
I've also been spotted many times because of the lime green Mickey heads (which I laminate and put on my backpack), and I've probably spotted 10 or 12 other people's, too. It is a pretty unmistakable sign. As people have mentioned on this and other threads, a plain lime green shirt or even plain lime green ribbon is just so common now that you can't really go by it.