Ok, maybe that's overselling it, but I had a thought that could actually address the problem with rules rather than the ole "DM fix it yourself".
First combat encounter after resting -20% xp.
Second combat encounter, standard xp reward.
All further encounters, +10% xp.
This does a few things...
It encourages the party to go for as long as they can.
It specifically encourages the powergamer to put off resting (there's a bonus for not resting!)
It addresses the fact that encounters at the beginning of the day are easier than ones at the end of the day (resources get depleted over the course of the day).
It rewards the book suggested amount of xp if the group fights the book suggested number of encounters each day (-20, +0, +10, +10).
Potential cons...
More work for the DM
-rebuttal: It's a trivial amount of extra work.
Could be abused if the group can reliably do well more than the intended 4 encounters a day.
-rebuttal: Some kind of bonus here is a good thing, that's the whole idea. However, if the group can reliably do significantly more than the suggested number of encounters each day, there is a probably a separate, more important, issue.
If the group insists on sticking to the "1 encounter, rest, 1 encounter, rest" they will fall behind the xp curve over the course of a campaign.
-rebuttal: With this system, resting after each encounter would be represent a poor choice and the game penalizes other bad choices so why not this one too?
So....already been suggested? Horrible idea? I'm a frickin genius? Any anyone have any thoughts?