[ESS] Shift Underline Produces '<-'

[Rich Shepard]

   I use emacs-23.2 on Slackware-13.1. A week or so ago I accidently pressed
shift-hyphen and saw the R equals '<-' presented. While this is handy when
entering expressions it prevents me from entering an underscore; for
example, value_var. Where can I turn off this shortcut?

TIA,

Rich