https://www.copyright.gov/title17/92chap1.html

A “work made for hire” is— (1) a work prepared by an employee within the scope of his or her employment; or (2) a work specially ordered or commissioned for use as a contribution to a collective work, as a part of a motion picture or other audiovisual work, as a translation, as a supplementary work, as a compilation, as an instructional text, as a test, as answer material for a test, or as an atlas, if the parties expressly agree in a written instrument signed by them that the work shall be considered a work made for hire.

I think I was probably misremembering my experience as a software contractor, in that I don't think I ever signed a single contract that did not include the "work for hire" stipulation.

Ironically, it seems far from clear that "work for hire" even applies to software: http://www.lexology.com/library/detail.aspx?g=59a4a8c4-c446-...