Here are a few pagraphs that I plan to add to my Academic Legal Writing book on this subject; comments are welcome.

Over two hundred student articles cite the online Wikipedia encyclopedia. Unlike with most encyclopedias, anyone is allowed to create Wikipedia entries, and generally to update existing entries. An unorthodox approach for an encycloped, but the theory is that (1) those people who want to spend time writing entries tend to be knowledgeable, and (2) even when they err, the errors end up getting corrected by others.

Surprisingly, the theory works, most of the time. Wikipedia entries tend to be relatively accurate, probably no worse and possibly better than the typical newspaper article. (This is especially so given that many newspaper articles are written by generalist reporters who are relying on hastily assembled materials from others.)

Nonetheless, while Wikipedia may sometimes be a good place to look, I advise you not to stop looking there. Instead, find the original sources that the Wikipedia entry's author relied on — they'll often be cited in the entry — and read, quote, and cite them.

First, that's the standard procedure you should use for intermediate sources (including, as I said before [earlier in the book chapter], newspaper articles). Second, whether or not Wikipedia is more reliable than the typical newspaper article, many readers will assume that it's less reliable; citing to it may thus decrease your credibility.

UPDATE: I at first noted that "I don't feel the need to mention that Wikipedia's contents may change over time, since I endorse citing to original-source Web pages, while recommending that the author print, save, and possibly even post and link to a copy of the page as of the time the article is written." D'oh! Forgot all about Wikipedia's change tracking system, which will let readers see the page as of any particular date (usin the oldid= feature). Thanks to Dan Lewis and RichardP for pointing this out.