I appear to have been invited to join the ACM. Everything looks entirely and genuinely legitimate, even down to it arriving as a snail-mail pre-membership package. The URL and mailing address in the package seem to check out as real and official. Not very sure how to proceed at this point, but it'd be a hell of a thing for the ol' resume if all goes well.

http://en.wikipedia.org/wiki/Association_for_Computing_Machinery