[quote=So Boerd]
Fabi is the vocative of Fabius, as Brute is the vocative of Brutus in "Et tu, Brute?"
[/quote]

Unless I am mistaken, you're supposed to call Patricians by their [i]cognomen[/i] (or [i]praenomen[/i] + [i]cognomen[/i]. [i]nomen[/i] or [i]praenomen[/i] + [i]nomen[/i] are resevered for addressing [i]homo novus[/i].

http://www.novaroma.org/nr/Using_Roman_names
w.vroma.org/~bmcmanus/roman_names.html
http://homepage.usask.ca/~jrp638/CourseNotes/RomanName.html <- This states that is should be [i]Praenomen[/i] + [i]Nomen[/i], since the cognomen was "not officially used until around the time of Sulla". However, I disagree on his example for two reasons. First, Cicero is a Homo Novus and therefore (according to the first link) would be called M. Tullius. Furthermore, I don't know why he used Catullus as his example for the support of [i]Praenomen[/i] + [i]Nomen[/i]. He is after Sulla (Catullus was born around Sulla's rule, but he wrote after it).

I would like to point towards Chapter 5 of Sallust's [i]The Catilinarian Conspiracy[/i]: "[b]L. Catilina[/b], nobili genere natus, fuit magna vi et animi et corporis, sed ingenio malo pravoque."

[i]praenomen[/i] + [i]cognomen[/i]