Is the plural "bosoms" an acceptable word? Or is it always "bosom"?

Under its entry for ‘bosom’, the OED has ‘pl. In recent use, a woman's breasts. colloq.’ with the earliest citation dated 1959. The OED would appear to have missed the references in Peter’s answer.

http://www.google.com/search?q=%22her+bosoms%22+site:gutenberg.org

In short, there are a few examples which are unambiguously referring to multiple bosoms of a single female and which are old enough to be out of copyright. They include poetry from 1888, pornography from 1898, and a novel from 1840 (albeit by someone whose poor prose led to the naming of a prize for bad opening lines).