If half of their friends also have their fly open, shouting it might be a better option.
And to be honest, I'm getting slightly tired of the community's tendency to stretch metaphors way beyond their applicability. Mine was mostly meant to add a touch of humor. Pants, flies, and underwear aside, auto-hijacking the Twitter accounts of everyone on your network is just dickish (no pun intended). Especially when the belt store is closed to everyone except the pants-manufacturers ;)
Regarding private/public notification; Posting to your own account is an easily understandable visible demonstration of what's possible and why you should care - a DM from another account, or an @ just won't have the same effect on the user.
(I have said this elsewhere in the thread, so apologies for repetition)
I have been informed that it's possible to send a DM to yourself, so I'll add that as an option shortly as a nice alternative.
I did tell my friends and family that a tool (Firesheep) has been released and what is the impact in layman language, and inform them to be careful when browsing through open wifi networks.
Yes, it's boring; yes, it's intellectually lazy and useless. No, it's not going away.
Point is, you don't know, so don't assume.
They walked into a glass building (unencrypted wifi location), unzipped their flies, sat down, and are totally unaware they did so. Someone would have noticed eventually - would they rather be potentially gawked at behind the one-way mirror that's always been there but they never noticed (WireShark), or told, allowing them to cookie-block the ones who do so?
As to the volume of the telling, what if near enough to everyone were doing this? You would probably become the crazy person at the intersection, telling everyone the end of privacy is near, repent and use SSL. But at least this way you can demonstrate that yes, their flies are down, in a way they can see, rather than having to take your word on it.