Elixirs will be featured in Diablo III, but little is known about them.
Strength and Dexterity were the only elixirs seen to drop in the Blizzcon 08' build. As of BlizzCon 2008, these provided a temporary, 300 second bonus to that one attribute.
A Vitality and a Willpower elixir were seen to drop during the gameplay trailer. It is suspected that these would give a boost very much like the known values of the Strength and Dexterity elixirs. It's not known if there will be quality levels of elixirs that would provide a longer or larger (or both) buff.