I've expressed a lot of concern about the Yankees blowing the division lead here. However, unlike most of the teams who have blown huge leads (1951 Dodgers, 1964 Phillies, 1978 Red Sox, 2007 Mets, etc.) the Yankees would still be in the postseason. I really don't buy into the whole momentum factor heading into the postseason. The 2006 Tigers were not very good down the stretch, blowing the division on the final day of the season, but still got in as the wild card. After losing their first playoff game they reeled off seven straight wins. The 2007 Phillies played really well down the stretch, winning the division on the final day of the season. They got swept in the first round.
If the Yankees get in as the wild card, due to the fact that they're still a really good team (and the crapshoot element of the posteason), they can still win the World Series. If that happens Yankee fans wouldn't care about the blown division.
However, with that being said, I will be very disappointed if the Yankees don't win the division.