As mentioned previously, there's a version of Westlaw optimized for use on small devices like smartphones. Not to be outdone, LexisNexis has a free app for using their system on an iPhone or iPod Touch. You can find the app by going to the iTunes store and searching for LexisNexis.

The app allows you to read or Shepardize a case on LexisNexis when you have a citation. The Shepard's feature is very limited, however, and doesn't include links to citing cases. So you can find out whether or not a case is likely to still be good, but you can't read or even get the citations to the cases that have reversed, questioned, criticized, or explained it.

See Legal Geekery for a detailed review of the app.