The NYTimes' Ben Shpigel examines Rollins' slump on their Bats blog , though his analysis could use more depth. The post is worth a read for Chris Coste's apt gambling analogies, though.
Bottom line: Rollins' walk rate is way down from last year, but nearly in line with his MVP season's in 2007, and his contact rate has remained solidly between .88 and .90 since 2004. His BABIP is just .218, though (career .300), and the culprit is bad luck and a diminished line drive rate. Essentially, he's hitting too many fly balls instead of line drives.
The steals are also down (and he's been caught an uncharacteristic five times in 16 attempts), but remember Rollins stole 30 bases after July 1 last year, so he can make up for the slow start there pretty quickly once the hits drop in.
There's no guarantee Rollins figures out why he's getting under the ball too much, or that his bad luck reverts to normal in the second half. But given his track record, the former is likely, and the latter is a risk you take with any player. If you can get Rollins for 85-90 cents on the preseason dollar, do it. That means if you need steals and have to move a Kevin Youkilis or Nick Markakis, make the trade, especially given the scarcity at shortstop this year.