APB was first heard of in 2005, a collaboration between Realtime Worlds and Webzen, due on PC in 2007 and Xbox 360 in 2008. Now the Korean publisher has been replaced by EA, the PC version is due in "early 2010" with closed beta testing starting in August, and "we have not announced a console strategy" (although 360 is certainly still on the cards). At least we can be pretty sure that it's not turning into GTA Online after all. As for why we haven't seen it yet, it's probably best to give the benefit of the doubt. Realtime very much kept its own counsel during the development of Crackdown, and that worked out quite nicely.
APB shares Crackdown's entirely open city setting, and its loose structure: there's no narrative as such, just a collection of Enforcement and Criminal factions offering dynamic objectives. If Crackdown was about physical freedom, then APB is about freedom of action and personal style. Going against the online grain, it actually has a lesser focus on character advancement than its offline predecessor.
Realtime wants gamers to earn their names in APB through their skill in combat, and creativity with its extensive and hugely powerful customisation tools, not the amount of time they invest. Celebrity is a core concept - one reason for the "quite personal" 100-player limit per city, although you'll be able to switch cities at will - but you'll earn it by being the coolest, the quickest or the most cunning, not the most powerful or most dedicated.
It's the combat side we currently know least about. APB is a third-person shooter and vehicular action game; car theft and territory wars will play a big part. Its RPG elements are apparently very limited. A sense of character advancement is a must, of course, but it's mostly achieved through unlocking cosmetic options, weapons, vehicles and usable items, and upgrading weapons and vehicles over time from a central pool of tokens. There will be ways to slightly increase your health, say, but Realtime is insistent that there will be "no arbitrary stats", and strict limits on weapon and item loadouts, to keep the game balanced for all players.
As for how conflict comes about, that will largely be up to the players. What you do, and how many of you there are, decides who you fight. You might, as one of the enforcers, take a mission from an NPC faction (like the Praetorians who defend the financial district) to take some territory off the criminals. As a criminal, you might steal a car to sell for profit, triggering its car alarm which will then summon player enforcement to respond and bring you to rights.
Your actions feed into APB's live, lobby-free match-making system which will pair up individual or group face-offs. In many cases this would be a straight one-on-one, two-on-two, four-on-four confrontation, but it doesn't have to be equal numbers. If the game judges that one skilful and/or well-equipped player is a match for three hapless noobs, it might set that up - or if a gang of criminals evades police capture for over an hour, say, they might end up with multiple groups of enforcers on their tail.