I am using 6.x-1.0-alpha2 and find that a CSV feed will do a full refresh even if the feed file has not been modified. The root cause is in _parser_csv_parse_feed() where the item timestamp is set by calling time(), on line 110 of parser_csv.inc:

$timestamp = time();

This means that the hash comparison in function _feedapi_invoke_refresh() is always false. A quick hack to the above line of code seems to treat this:

$timestamp = 1;

NB I have looked at CVS. At the time of writing, the latest revision is 1.2.2.9 and it does not appear to address this bug.