I have more or less gotten this question answered in another thread so I'll just sum it up and add a few links for the benefit of future readers:

Cicchetti seems to be just the thing.

Unlike aperitivo it is served all day.
There are plenty of bars all over Venice.
Very casual - standing up at the bar mainly.

Link to post with good links to list of cicchetti bars:

http://www.flyertalk.com/forum/27333826-post32.html

Links to other relevant posts:

http://www.flyertalk.com/forum/27336230-post37.html

http://www.flyertalk.com/forum/27337096-post41.html

Also if someone absolutely needs to have pizza in Venice, there is a fast food pizza chain that seems to serve tolerable pizzas (napoli style):

Rossopomodora: Calle Larga, S. Marco