Is there a PVP addon out there that will allow me to automatically target whomever may be targeting me, even if they have yet to attack?

Sure would be a nice addon.

Nope. That would be automation which is not allowed.  You might want to look for a addon that will allow you to see who's targeting you, but you still would have to target them manually.

If they haven't used an ability on you, I don't believe there's a way for an addon to know they have you targeted -- there's no "someone targeted you" event the server sends.  The information they see about you comes from the server, not from your client.

I know at one point with Xperl it would let me know if someone was targeting me or I had some form of aggro.

TipTac will allow you to see if someone is targetting you, but only if you mouse over them.

CowTip can also show if something is targetting you simply by mousing over that unit.