Nick Konkle was the guest on a QGN Twitch cast on Thursday evening and he pretty much said the Imperial City will not offer much PVP except for wiping out the other factions that don't leave voluntarily when the city changes hands. If your faction does not control access to the city, you will not be able to enter it. Access to the city is determined by whatever faction controls the most (not necessarily all) of the 18 total keeps. If enough of the 18 keeps change hands to give your faction control of the city, your faction can then enter the city and wipe out players from the other factions who will no longer be able to enter into the city. Once your faction controls the city and you die while inside the IC, you can respawn inside it and bypass the sewer entrances, thus circumventing gankers who may be camping them. If you are outside the IC, you cannot respawn into the city at the shrines and you must enter the IC via sewer pipes.
It sounds like the Imperial City will mostly be a PVE zone. The discussion begins around the 12:45 mark.
http://www.twitch.tv/questgamingnetwork/b/550879608
ETC #1: Corrected the number of keeps in play (all 18, not just the central 6) that determine control of the Imperial City. Thank you,
@ZOS_BrianWheeler for providing this important correction.
ETC #2: According to Brian Wheeler near the bottom of the second page of this thread, you can only enter the IC via sewer pipes which are located near keeps. If you are in the IC, you may respawn at shrines in districts controlled by your faction. If you are elsewhere in Cyrodiil, you will not be able to respawn at the shrines in the IC and must use the sewer entrances to the city.