To a large extent, the "5-minute workday" is about how the group choose to play the game, rather than something hard-wired into the rules.
It is likely that some groups will see no change: in every encounter the characters will blow their most powerful (probably daily) abilities in order, will then use whatever healing surges are required to return to full health, and will retreat and rest as soon as one resource or the other is depleted. The DM will compensate with tougher encounters, and the 5 minute workday will be back in full force.
Other groups will consciously decide to avoid the 5 minute workday by intentionally conserving their powers longer, and will hail this as a triumph for the system. Of course, they could do exactly the same in 3e through the same determination and by working with the DM to achieve this end.
Probably the largest group, though, will see some measure of a shift towards a longer adventuring day, without really doing much to change their playstyle. How much of a change will depend on how well the designers have achieved this goal, and remains to be seen.
Personally, had I been designing 4e I would have been tempted to go with a pure per-encounter balancing mechanism, thus eliminating the issue entirely. Alternately, I might have gone for a mix of per-encounter and per-adventure resources (say, split the hit point pool in half - half returns automatically at the end of the encounter, and half can only be regained through extended bed rest for several days to weeks). But that's just me - it will be interesting to see how the rules we're actually getting will play out (and how they will play out a year or two from now, once all the quirks and exploits have come to light).