# of teams in top 3 (including teams tied for 3rd) in W-L% on April 18 to make playoffs
2008: 3/5 (Cubs, Brewers, White Sox)
2009: 1/3 (Dodgers)
2010: 3/3 (Yankees, Rays, Twins)
2011: 1/3 (Rangers)
2012: 4/5 (Rangers, Nationals, Tigers, Cardinals)
2013: 3/4 (Braves, A's, Red Sox)
Over the past 6 seasons, 15/23 teams made the playoffs, including 7 out of 9 in the last two years. That's probably a trend that will continue because of the additional wild card teams added in 2012. In 2012, two of the four teams were wild card teams, and the Cardinals (who nearly made the World Series) were the 2nd WC team with 88 wins.
The top 3 teams today are the Brewers, A's and Braves. The Brewers are the only surprise, and are actually the only real surprise in the top 10 (4-10: Yankees, Giants, Dodgers, Cardinals, Tigers, Rangers). The team with the #1 record on April 18 has made the playoffs each of the last 2 years (Braves, Rangers) and 3 of the last 4 (Rays, Yankees tied for 1st in 2010).
A team's record on April 18th seems to be a good predictor of whether they'll make the playoffs, especially with an extra wild card slot in each league. I don't think the Brewers will end up with the best record or even in the top 3, but I think they will at least be competitive for a wild card spot.