Recently in a blog, the comment I'm sure we've all read was that the wizard scored a crit with a fireball (Wizard/Warlord, I think) that killed just one troglodyte. There has been some confusion and outrage that fireball might require a hit roll and might not be an area effect. I have a theory on how spells are going to work in 4E, so I hope this is what we're seeing.
The wizard memorizes a spell (fireball). The spell has three effects, the once-per-day, once-per-encounter and at-will. Just hypothesizing, the once-per-day fireball effect is 1d4 per level (character level?) to an area. The once-per-encounter is a ray of fire, roll to hit, 1d4 per level to one target. The at-will is a close-range or melee range fire touch attack for 1d4 damage +1 per level. Using the per-day ability uses up the spell completely, so the at-will and per-encounter are not available. Using the per-encounter uses up the spell temporarily, so the at-will isn't available for the rest of this fight.
This three-level effect per spell would make sense from the descriptions we've been given and also make for better tactical play as the wizard has much more reason to hold back his spells than before.
It could also open up some other interesting uses, for instance the at-will bonus to Charm Person could be a something for use outside of combat, like a bonus on social rolls.