Pursuant to a Rule from the National Labor Relations Board, most private sector employers are required to post notice of employee rights under the National Labor Relations Act. This Act is what gives many private sector employees the right to form and join, or refrain from joining, labor unions.
The new rule requires that a poster be hung in locations similar to where other such notices (under the FMLA, FLSA, etc.) may be posted. The Rule now goes into effect April 30, 2012, after the original effective date was postponed by the NLRB.