This is the conclusion of a research study to be published in the next issue of Internet and Higher Education. I've emailed the authors to see if they're planning on depositing the paper in their Institutional Repository... It's only a pre-print right now so I can't give you a complete citation, but if you subscribe to Science Direct you may be able to access the article here.
UPDATE 2007-02-06: You may have better luck with the DOI link
The study is quite small (N=27) and the number of citations compared is also (N=72). My summary would be "students can get good (trustworthy, high quality) citations by searching through their library or through Google Scholar, but if they search only Google they often get junk and don't realize it". The specific metrics they used for quality were authority & reputation, objectivity, academic rigor, and transparency. While these criteria are relevant for academic work, they may be less so for the general public.