Writing about the Veracity of Online Sources

Via metafilter, an interesting discussion of the veracity of an electronic source, an alleged e-mail home from an apparent Special Forces Major or Lieutenant Colonel in Iraq. The discussion is politically charged, as is the letter, and makes me ask, how do we address concerns about the veracity of such electronic sources when students want to bring them into the classroom and into their written work? Do such letters, without any real verifiable source or annotation history, serve only to preach to the (pro-war or anti-war) converted? This would seem to be a case where the politics and the language (remarkable, to me, for its odd juxtaposition of context-dependent and context-independent terminology in an e-mail, as if the author was either not sure who would be reading the piece or else was not a member of the community to which the terminology belonged) muddle one another so much that I'd feel like telling any student who wanted to use it, (1) It's not a reliable source, and so won't help you to support any argument you make, but (2) you could write an interesting paper arguing for or against its reliability. I know there are plenty of teachers who do a lot of classroom work with evaluating online sources (on preview, there's now another Metafilter story about online information literacy) in service of writing more effective essays; have any folks here built essay units of their own entirely around the concept of the veracity of online sources? (In other words, made it the content of rather than a prerequisite for an extended writing assignment?)