I noticed while watching 300 that the smaller spearheads were used for slashing at certain times during the film. And while I know that 300 wasn't meant to be historically accurate, it did make me wonder if it could be used as both a slashing and piercing weapon.

Does anybody have any experience or ideas on this? Are the sides even usually sharpened on most spears?[/i]

Most spears have a sharpened edge, and would deliver rather nasty cuts with a good hit.
Tests have shown that a sharpened spear will easily cut to the bone, with a good, test cutting style stroke.

However, most spears are not made for cutting, and are significantly less lethal in this role.

Some spears ARE made for cutting, having longer blades and beeing mounted on stouter shafts. The partisan can be seen as a extreme development of these.
It is also quite possible that the viking Atgeir was a short shafted hewing spear. (see the Viking helbard thread for a discussion of this... http://www.myArmoury.com/talk/viewtopic.php?t=5813 )

However, one handed spears are not balanced for striking like shown in movies like 300, and would probably not cut very well if swung in that fashion.
Atgeirs would have been short enough to be used to cut in an axe-like fashion. (with a shaft of about 1 m)

A greek spear could be used for slashing in an emergency, but the style of greek combat is massed phalanx formation was not suited for slashing motions. The greek sword was very short, and even it was a last ditch weapon in battle lines. 300 was an action movie, and you saw how quickly the producers abandoned the whole phalanx idea for more exciting cinema. For greek spear use, best stick to overhand thrusts.

Overhand or underhand. Both methods are amply attested in Greek iconography, and both are applicable in close-quarters phalanx fighting.

I would agree with Mike, that overhand stabbing was the predominant motion used while in phalanx formation. Underhand spear handling certainly shows in Greek combat imagery, but when a phalanx formation is shown, the spears seem to be held overhand. Any kind of slashing motion would likely be quite difficult to do, if you were staying in formation.

Although not a usual tactic by any means, slashing with a spear would be an effective maneuver if you needed another second ro two in order to line up a killing blow. Or simply as a means of disabling someone.