i didnt see anything about flanking in the how to play or DM guide in the playtest. *shrug*
As far as I know because I don't have the playtest and am only hearing it second hand is that there is no flanking or attacks of opportunity or anything like that. What was suggested (I don't know if they stated this in playtest material or it's just the way a lot of DMs were doing it.) was that in situations like that you would give the attackers the "advantage" mechanic they came up with in the new addition where you roll two d20 and take the result of the best one.
I think they put it in the DM's hands to decide when a lot of the advantage/disadvantage situations happen...so it's pretty much by DM fiat, which again I'm not sure I like that much.
I'm not too big on their being no attacks of opportunity or flanking myself. It's way easier for some opponent to go past the front line people and attack the more squishy party members like the wizard. It's even worse considering pretty much every character in the game basically gets spring attack for free in this addition. Sure they said there will be abilities that allow for defenses of that, but I don’t really like the ideal of having to take a special defensive ability just to stop people from passing somebody.
I’ll admit the advantage/disadvantage mechanic they came up with actually seems like it might have a lot of promise. In some ways it's not too bad. The downside I think is that with the number of playtesters that like it they are going to force that mechanic to be everywhere in the new addition. I can almost sense it coming already. I think it works well as some very occasional and neat thing that comes up every now and then, but I have a feeling it's going to get totally ingrained in just about everything in the game, and I'm not really sure I like the implications of that mechanic wise or that it will make things less compatible with every other edition. Not to mention I think in some ways it's almost too gamist, but at least not that bad. What I'm even more worried about is it might be too simple. I like having various modifiers I can add to rolls for the situation and that mechanic seems like it will make a lot of modifiers pretty much the same as a one size fits all for a lot of things.
Overall they do seem like they are trying to make the game simpler. I guess that's good for the people that want that, although that also carries with it repercussions of it's own.
It should also be noted that this is only the first playtest and not only aren't the rules far from set in stone yet, but a lot of the rules they will have aren't even been tested out now. I wouldn't be surprised to see things like flanking and such as a latter addition or maybe as a module.