As long as the article on Wikipedia is backed up my credible sources (Which alot are now) I see no issue with using it as a source of reference
It really depends on the site. Through my college courses, the teachers I had for literature and composition classes would only accpet online references from certain sites, or sites that bear a certain appendix to them (.org, .net, etc - .com was out of the question). The reference site itself had to be from a respected source like Encarta or CNN or whatever, but we were typically limited to only two online references - one if it was a major end of session paper or mid-term paper.
That didn't stop me from using online libraries, though - there is no difference there, IMO. A book is a book whether it appears on the screen or on paper.