As far as I can tell from the info about the pregens, I don't see the 90min day in the same regard as 3e 15 min day. Even if we can't say that much without knowing more about the full class powers progression through levels, I'd say
FOA it is mainly a DM attitude to avoid that sort of problem. Supposing a not-lame and quite experienced group of players should know how much of their resources to spend every encounter, depending on the task they're going to carry on and if they roughly know they kind of threts they're going to face, the DM should be able to lay down his adventure in a way it could span through several encounters without the need for the players to take an extended rest. The wizard/cleric "I need my spell list back" and the fighter's "I'm not going to push on since we have no healing left" issues should be far less frequent in 4e, given the "per encounter" factor and the PC classes generally being able to not relay heavilly on other classes to survive.
Consider a major shift in gameplay style is occuring in 4e.
In 3e players where supposed to deplete a consistent part of their resources to face a balanced encounter, meaning they would usually cast healing, buffs, use magic items and the like, as soon as the need arises. Since the very game mechanics implied this sort of behaviour, it was just like the 3e book said to you "don't expect to survive more than 4 encounters in a row, given you're not fighting crap".
OTHO healing/buffing/supporting is going to be there every encounter in 4e, that not meaning you'll be deplating your resources. Leaders can easilly spread out healing surges/bonuses to defenses or saving throws or attack rolls/other kind of buffs on their comrades, all of this using at-will or per-ecounter powers (wich of course they regain after a short rest). You can foresee a huge boost to your party effectiveness coming from clerics/warlords/bards. Also rogue's are said to gain a lot of combat advantages coming from other party members' powers, bringing much more damage on the battlefield.
If I'm getting it right daily powers, wich would likely be powerfull enought to turn a desperate situation into victory, are not key to stop your adventuring day, leaving this role to surges and other healing tools.
Giving you regain sugers, action points and per-encounter powers after a 5min rest (wich I don't see why the DM should be going to interrupt every time the party tries to take it), an experienced and tactically effective party could easilly take a lot more than 4 encounters. Considering there's not any real reason for the DM to throw every time a tough encounter on his players in a tipical adventure, the encounters-in-a-row level can quickly go up to tens (a balanced dungeon, ie, would feature many encounters below the party level, a bunch of balanced ones, some tough ones and maybe the threting final boss rush).
What I'm courious about is the opposite problem. I think the single encounters are going to be quite fun and easy for a DM to design, but the PCs being always able to relay on the most part of their resources (per-encounter powers, powers coming from MIs wich seem to be per-ecounter too, a reasonablly solid number of surges, buffs) is going to make it hard to predict the general outcome of a series of subsequent encounters.
If you throw many low level encounters at the party (and I mean something like 1 or 2 levels below it), the players are going to make it quickly, without using dailies, maybe using 1 or 2 surges (wich of course they regain when the encounter is over), vitually being able to push on all the day long not only without the need to rest, but even not having to fear the next battle.
Balanced encounters (ie encounter level=party lvl+/-1) will be exciting of course, but here goes the same. Maybe the players are going to use more surges, one or two dailies if it is really needed, but if they know how to effectivelly use Action Points and per-encounter powers they could push on a lot before the need to take the 6h rest.
So, IMO, the only effective way for the DM to put a strain on the party is going to be having it face dangerous threats (higher lvl mobs, elites or solo), wich are going to be the "we are going to camp" or TPK moments of your sessions.
Interestingly, even with all the effort to erase the save-or-die rolls (and I'm using the broader sense of it), go ask Andy Collins (at least I think it was him) about his beloved Im-going-to-kill-you-anyway-high-lvl-solo black dragon. As he wrote, if the wizard hit it with the sleep spell it was quickly taken care of.
Wizard casts sleep -> dragon is hit -> the party attacks en mass -> daily powers -> players using action points to hurt much more -> if the dragon does not save it's doomed -> if the dragon is lucky enought it saves to bring some destruction on the battlefield -> the dragon probably dies in the next few turns anyways. Luck will be a major factor this time around, more than in 3.X.
And we are talking about 1st lvl pregens here. So I sense hard times for the DMs out there, either risking to take a TPK instead of a tough encounter in the effort of giving some hard times to your now-powerfull low level players, or seeing your uber-encounters being quickly wiped out thanks to some lucky roll by the players (or unlucky ones by you). This is far better than save-or-die effects, but at least you usually couldn't use them on opponents higher in level than you (while it seems spells and other effects still retain some meaningfull chance to hit in 4E, regardless of level difference).
Theese are my thoughts, based on the bits of info we have on 4E so far, so I don't really fear the 15min (or 90min) adventuring day issue, since in 4E it will more likely occour because of the DM not designing the encounters/adventures the right way or the players being too carefull/not smart enought to manage their resources the right way. Sorry for the long/confusing post too (I actually hope someone read it #_#)