A couple of thoughts...
If Nobility is a feat, how do you handle nobility as a reward? Often in game, PCs are granted nobility as a reward for a quest. Do they suddenly have the Noble feat? What about a PC marrying into the nobility?I can think of countless situations in which the concept of a characters Nobility can exceed the simple mecanics of a feat.
The best way to handle it is to have seperate mechanics for Noble Npringing, and Nobility itself. Noble Upringing should be a background feat that grants additional starting funds, and situational skill bonuses. Nobility should be a rank based on material wealth, level, aand land ownership
Another idea is that those of Noble blood are in fact ordained by the gods, granting some special or spell-like abilities. aybe all nobles are Aasimir...