if u'r using a database a solution is to add a timestamp to the user and whatsnew table..
when the user logs in, retrieve the timestamp and update the timestamp to the current date/time.. then create a query that retrieves all records from the whatsnew table that are newer then the retrieved user timestamp.. e voila...
u can also do this with cookies, but u still have to create a timestamp to the whatsnew table.. feed a cookie with:
setcookie("lastlog", time()); (time() is the value (php timestamp) of the cookie, not the expire time).. do the same as above...
i think the first solution is better, because users can delete the cookie ...