I just tested this in excel 2007, and it works correctly for me. However if there is a leading space before the quotes, then it does not parse comma correctly.
The only workaround I've found that works is to eliminate the = when the field contents include a comma.
This may mean that there are some fields that are impossible to represent exactly "right" in Excel, but by now I trust no-one is too surprised.
This wouldn't appear to be a bug according to RFC 4180. The correct way to quote the field is to quote the whole field and then double quote internal quotes. So "=""field, content""" tools.ietf.org/html/rfc4180
WARNING: Excel '07 (at least) has a(nother) bug: if there's a comma in the contents of a field, it doesn't parse the ="field, contents" correctly, but rather puts everything after the comma into the following field, regardless of the quotation marks.