And, if I type in a spreadsheet from scratch, with numbers in the cells:
if I started Excel for Mac 2011 in a locale with . as the decimal separator and , as the thousands separator (US locale), and save it as "Windows CSV", it writes it out with "," as the field separator, with "." as the decimal separator, and without thousands separators;
if I started Excel for Mac 2011 in a locale with , as the decimal separator and . as the thousands separator (customized, based on US locale, with the decimal and thousands separator changed), and save it as "Windows CSV", it writes it out with ";" as the field separator, with "," as the decimal separator, and without thousands separators.