All by TV ratings and also somewhat on geography.
You won't see a West Coast team at 1PM Sunday if they can help it.
The two things we do know are that FOX and CBS each get one game on Sunday the first weekend, meaning that one AFC and one NFC game each day. The same is also true the next week as FOX and CBS get one game each on Sat and Sun.
NFL Playoff Schedule
Best guesses for Week 1:
AZ @ 8PM SAT
PHI/MIN/DAL @1PM SUN
CIN @4:30PM SAT
NE @4PM SUN
Week 2 guesses:
INDY @1PM SUN
SD @8PM SAT
PHI/MIN/AZ/DAL @4PM SUN
NO @4:30PM SAT
I'll play this game

A little walk through history shows that the Sunday games have alternated the conference playing at 1 PM every time all the way back to 1999.
This year it would appear that it's the NFC's turn to have a 1 PM game with an AFC game at 4:30 PM on that day. Presumably that this is done to keep things balanced between FOX and CBS.
With that in mind, it looks like Arizona will be playing on Saturday, unless they sneak into the #2 seed. (1 out of 8 scenarios). In 6 of the other 7 scenarios they will be playing the Packers. I don't think they would put GB-Ariz on as the 8 PM game. I also doubt the Bengals would be put in prime time two weeks in a row on NBC. I DO think that regardless of matchup, NBC wouldn't mind putting the love to hate Patriots on in that spot. The only opponent I can see getting the Bengals on at 8 PM would be the Steelers.
So with that:
If Arizona isn't the 2nd seed:
Sat 4:30PM - @Arizona
Sat 8 PM - @New England
Sun 1 PM - @Minn/Dal/Phil
Sun 4:30PM - @Cincinnati
If Arizona is the 2nd seed, I think that changes things. If Arizona gets the 2nd seed, the other games will be Dallas-GB and Minnesota-Philly.
I think with these match-ups, there's a much higher chance that the Saturday night game becomes an NFC game. As much as GB and Dallas have big national followings, I gotta think that ultimately NBC will want to have Favre and that the NFL would be willing to listen to that bit of lobbying. Suddenly, the Bengals game becomes clear Saturday afternoon fare.
If Arizona IS the 2nd seed:
Sat 4:30PM -@Cincinnati
Sat 8 PM -@Minnesota
Sun 1 PM -@Dallas
Sun 4:30PM -@New England
So there you go
-QG