Jessica Enders wrote an article on [A List Apart][2] about three studies she did to determine whether "zebra stripes" are helpful. 

The first study, described in an [earlier article][1] (which this answer referenced originally), tested users' ability to read and interpret data in a simple table. The second study was similar to the first, with an improved methodology. The third study attempted to determine whether users tend to have a subjective preference for striped tables.

> ### The recommendation
> 
> The results of the three studies
> conducted to date suggest that the
> safest option is to shade the
> alternating, individual rows of your
> table with a single color. Taking this
> approach is likely to ensure that:
> 
>   - task performance is better, or at least no worse, than with other table
> styles, and
>   - the aesthetic sensibilities and subjective preferences of the majority
> of your users are catered for.
> 
> If zebra striping of this type cannot
> be done easily, then ruling a line
> between each row may be the next best
> option.


  [1]: http://www.alistapart.com/articles/zebrastripingdoesithelp
  [2]: http://www.alistapart.com/articles/zebrastripingmoredataforthecase/
  [3]: http://formulate.com.au/research/data-tables-follow-up/
  [4]: http://twitter.com/patrick_mc/status/37891482346205184