NFL Rivalries

[editor's note, by Skin Patrol] For research and because I like the poll.
In a recent article in Sports Illustrated, there is a section about multiple NFL Rivalries -- Tim Layden writes about the Bengals and Steelers, Bill Syken writes about the Panthers and Bucs, and Nunyo Demasio writes about the Jags and Colts.

My question is two-fold: How long does it take to develop a true rivalry?  And more importantly, How can you not even mention the [fill in NFC East team here] and [fill in other NFC East team here]?

The Bengals were established in 1968, and the Steelers in 1933.  Of the 72 regular season meetings between the Steelers and Bengals, Pittsburgh has won 42 of them (.583).  The Steelers are 1-0 in postseason play.

That sounds almost legit.

The Bucs were established in 1976, and the Panthers in 1995.  Of their 12 regular season meetings, Carolina has won 7 (.583), and they have never met in the postseason.

Is it really possible to have a rivalry, when you have only played the other team 12 times EVER?

The Colts were established in 1953, and the Jags in 1995.  Of the 11 times the teams have played, the Colts have won 9 of them (.818).  They have never met in postseason play.

Is it really possible to have a rivalry, when you have only played the other team 11 times, and the record is lopsided to the tune of an .818 winning percentage?

The Redskins were established in 1932, and have played the Cowboys (1960) 93 times, with the Cowboys winning 55 of them with 2 ties (.604).  They have met in the postseason twice, with the Redskins winning both of those meetings.  
The Skins have played the Eagles (1933) 144 times, with the Skins winning 75 of those times with 6 ties (.543), and are 1-0 against them in the playoffs.  
Washington has played the Giants (1925) 147 times, and the Giants take that series with 83 wins and 4 ties (.580).  They are 1-1 in the playoffs.

That is just the Redksins record vs. other NFC East opponents.  I was too lazy to look up the Giants vs. Eagles, Cowboys vs. Giants, etc.

So, in your opinion, what is the biggest rivalry in football and for what reason?