I actually don't think this is totally accurate. Possum is an alternate pronunciation/spelling of opossum. According to this thinly sourced Wikipedia article:
The common name "possum" for various Phalangeriformes species derives from the creatures' resemblance to the opossums of the Americas... However, although opossums are also marsupials, Australasian possums are more closely related to other Australasian marsupials such as kangaroos.
Can anyone confirm this one way or the other?