The National Association of Colleges and Employers offers valuable resources regarding writing letters of reference. Take a look! Keep in mind these sites offer general guidelines and should not be construed as legal advice for any specific situation.
Writing a Reference Letter http://www.naceweb.org/public/kaplan.htm
Discussion of the issues surrounding letters of reference.
Suggested Guidelines for Writing References http://www.naceweb.org/public/reference1.htm
Itemized guidelines to observe when writing references for students.
Faculty Reference Letter http://www.naceweb.org/about/public/formfacref.htm
Contains an example for use when creating a letter of reference for students.
A Faculty Guide to Ethical and Legal Standards in Hiring http://www.naceweb.org/committee/whitepapers/faculty.htm
Discussion of the ethical and legal standards involving students obtaining employment.