I think anything on the internet should be taken with a grain of salt. Wikipedia has been at the butt of many late night jokes. But curiously, if you look up something on Wikipedia, say "string theory" for example, you'll see at the bottom of the article all the references that was used to compile the piece. If you Google "string theory", you'll find many of the same that was referenced on Wikipedia.
With this in mind, I think Wikipedia is as serious as anything else on the internet. However, If you need to turn in a paper and you list "Wikipedia" as your reference, then I don't think your paper will be taken too seriously. But if you list, "Kaku, Michio (April 1994). Hyperspace: A Scientific Odyssey Through Parallel Universes, Time Warps, and the Tenth Dimension. Oxford: Oxford University Press. p. 384. ISBN 0-19-508514-0." as a reference, then your paper will look much more serious (got that from Wikipedia BTW).