Crossbows
No crossbows have yet been seen in Diablo III. Bows are fairly common; they've been seen as dropped items in gameplay movies and are often used by monsters and NPCs, but no crossbows have been sighted yet.
Archer Character?
It is widely expected that the fifth character will be a specialist in bows and perhaps all types of ranged weapons, and that the development team is holding such item types out of the game until that character is revealed at BlizzCon 2010. Presumably if there are bows, there will be crossbows, since such items featured in Diablo and Diablo II. This is not yet confirmed, though.
Archer NPCs
Various NPCs have been seen using bows. Several wandering NPC warriors were seen using them in the WWI 2008 gameplay movie. These archers attempted to accompany Deckard Cain and the Barbarian through the dungeon, but were picked off one at a time by various monsters.
Leah is seen holding a bow in town in the Caravan screenshots released in August 2010. It's unknown if she ever accompanies the player into combat, or is just a sort of apprentice Rogue who only holds the bow for status points while in town.
Archer Monsters
Skeletal Archers are common enemies in Act One screenshots. It's unknown if there are higher level monster archers found later in the game.
Bow Images
The only bows seen so far have been equipped by monsters and NPCs. A few examples are included below.
Skeletal Archer in action.
Deckard Cain and a bow-packing Leah.
