Today Ireland play at Wales, and if they win they will be crowded Six Nations champions with an undefeated Grand Slam season. If they lose, they will find themselves in a tie with Wales, and, in a perfect world, the Welsh would win the title on the strength of the head-to-head victory (not quite perfect, of course, since the home field advantage is so huge in rugby, especially in Cardiff, you know Welsh choirs and all that).
But in international rugby, to the winner the spoils do not necessarily go. The Welsh will have to beat Ireland by 13 points or more, because in the case of ties, the winner is determined by 'points difference' in all five matches. So in reality the Six Nations will be decided by who was able to run up the score against Italy by the most points.
Remember 2001? Due to hoof and mouth in Ireland (among the cows, not the rugby players), the Irish finished their campaign in the fall, and in their final match faced English, who had juggernauted through the other four nations in the spring. An Irish win in Dublin would have left the teams level with 4-1 records, but Ireland had beaten Italy 41-22 away, while the English had enjoyed embarrassing Italy 80-23 at Twickers. They'd beaten Scotland by almost as much, and with a 143 point differential were forcing Ireland to beat them by more than they'd beaten Italy in order to be champions. In the end, Irealdn duly won, 20-14, but there was no suspense in the match, as there would have been had anything been as stake.
The Six Nations is often decided on PD: France over Ireland in both 2006 and 2007, although luckily they beat the Irish head to head both years. Or I should say fortunately they'd amassed a better point differential. Wouldn't be better if titles were decided on the field, and not by some guy in a blazer working a pocket calculator?