For the record, FO1 didn't have slaver tattoo, FO2 did.
The way it worked was that "good" people wouldn't talk to you or offer you quests/shop/other help, but you obviously got tons of money from enslaving people early on. I suspect it wont go to this extent in New Vegas, but just make it so that everyone knows you belong to the Legion (help from people who like them, hostilities from people who don't). Or it could be purely cosmetic.
I was thinking Joining the Khans in FO1 anyways I hope the tattoo in New Vegas works the same way as it does in FO2. I am sure their will be dialogue were you can use it to intimidate those that will not talk with you for being a slaver.
Wasn't that Tattoo unique to Metzgers band of slavers? I'm having a hard time believing there's a universally accepted brand used by all Slaver organizations. I would imagine each has their own distinct calling card, tattoo or other.
The Legion controls amries of people and alot of territory, their mark will be well known by everyone.