Still occurs in 1.6.2
Can confirm that it still happens in 1.4.4 pre-release.
In a new world, changes made near the origin of the world remained during the session, I ran 800 blocks away and modified a distinct landmark, and died. Upon respawning I could see my changes near origin, but when I returned to the landmark 800 blocks away it was untouched. I ran back to world origin and I could still see my changes at origin. I closed Minecraft and loaded the world again and only then did the area near Origin appear untouched.
@Raphael "Sometimes you run on a flat surface and you slow down for no reason" Maybe he doesn't know that, and it's the start of hunger
Sprinting ceases when the "sustenance" or hunger reserves have depleted and the hunger bar begins to jiggle and go down. You can then sprint again until 3 bits of hunger remain on the hunger bar. Vaguely relevant tip: You can sprint up a half-slab staircases
This no longer happens - it has been fixed or was never an issue. I was expecting passive mobs to spawn immediately on some newly generated chunks, but there can be quite a long delay. The short rendering distance for mobs didn't help either.