I have used a houserule similar to this (taking into account what Yakk said) in the past and it worked nicely as a pacing adjustment. Basically, it comes down to this: When the players want to rest, the DM decides if they have rested successfully. Some examples:

The players are tracking bandits through the forest. They're attacked by some bandid scouts and keep moving after the fight. It doesn't matter how long it takes them to navigate the rest of the path or if they get (or are unable to get) 5 minutes of rest (maybe while the party tracker is looking for clues at a difficult spot). Since the party decided to "keep moving", they get no rest if they're attacked again.

When they reach the bandit hideout, they see a group guarding the entrance and decide to retreat back into the forest and take a short rest (which also makes sense fluff-wise, since they just spent a good deal of time frolicking in the woods). Since the bandit guards didn't spot them, they succeed and get their encounter powers back.

After clearing up the hideout, the players decide to spend the night there before heading back to town. During the night, a group of bandits arrive and finds them camping there. Even if it has been longer than 6 hours, the DM decides that the PCs don't get any spent dailies back.

The efficiency of this rule depends on the DM and players, of course. It worked well with us because of the players, who would constantly save their encounter powers unless absolutely necessary (their reasoning was "what if reinforcements arrive?"), and this meant I could spring a second encounter with no rest and they'd still have some juice left.

Another thing to note is that just because the DM can rule that a rest needs more time to work doesn't mean he should. As Kurald said, in a dungeon crawl it would be pointless to force the party to wait too long to continue adventuring (the flip side of the houserule is that you can let them have a long rest in less than 6 hours if you want to).