Mo the caller, 'could of' is also heard in America.
This departs from the subject of this thread, but here is Webster's Collegiate definition of dialect:
1. a regional variety of language distinguished by features of vocabulary, grammar, and pronunciation from other regional varieties and constituting together with them a single language.

By this definition, 'could of' is merely incorrect speech. But listening to myself on a tape, my 'could've' is mighty nigh 'could of' (but more 'could'uv')- sloppy, sloppy!.