WHIP is an acronym for Walks + Hits over Innings Pitched, a way of measuring how many baserunners a pitcher allows. Obviously, the fewer baserunners a pitcher allows, the harder it is for the opposing offense to score runs. Over the past five years, the major league average WHIP is around 1.400. So Sabathia's 1.115 WHIP is very good, indeed. It's nowhere near record-setting, however.
Since 1990, eleven different pitchers have qualified for the ERA title (1 IP per team game, usually 162 IP in a season) with a WHIP of 1.000 or lower a total of twenty-two times. Since 1901, ninety-eight different pitchers did it 161 total times. It's possible to find different eras in baseball history by looking at the list. From 1901 to 1920, the deadball era, eighty of those 161 seasons were recorded. From 1921 to the start of World War II, when offense reigned, only one pitcher made the list (Carl Hubbell, 1933). The 1950's only saw two pitchers join. The so-called "Year of the Pitcher," 1968, had nine pitchers accomplish the feat. You get the idea.
It's kind of interesting that while offense in the late 1990's and 2000's increased to levels not consistently seen since the 1930's, the number of pitchers with a WHIP under 1.00 hasn't been impacted like it was back then. Presumably this has something to do with increased strikeouts; if hitters aren't putting the ball in play as much, they don't have as many chances of those balls falling in for hits, thus inflating the pitcher's WHIP.
Whatever the reason, there have been twenty-two sub-1.000 WHIP seasons since 1990. The list of names reads like a future Hall of Fame ballot, with a couple exceptions.
* - Cy Young Award winner
Pedro Martinez's 0.737 WHIP in 2000 is an all-time record. The only other pitcher to have a WHIP under 0.800 was Walter Johnson in 1913 (0.780). Addie Joss had a 0.806 WHIP in 1908, and Greg Maddux's 0.811 in 1995 is fourth all-time.
The major league WHIP leader in 2008 was Toronto's Roy Halladay. He issued 39 walks and allowed 220 hits in 246 innings, good for a 1.053 WHIP. The 2007 leader was Jake Peavy of the Padres at 1.061 (169 hits, 68 walks, 223 1/3 innings). Justin Duchscherer wound up 20 1/3 innings short of joining the list in 2008: he had a WHIP of 0.995 in 141 2/3 innings.
John Smoltz in 1996 got as close as you can get to a 1.000 WHIP without actually getting there. He allowed 199 hits and 55 walks in 253 2/3 innings. He was one out short of a 1.000 WHIP, but he had to settle for 1.001. Maybe I should've rounded?