You have to hand it to a group of researchers at Washington State University. They were able to find unopened boxes of major league baseballs from the 1970s for testing purposes. They were trying to find out whether there was anything to the assertion that baseballs of today are "juiced," manufactured in a way that makes home-balls easier to hit.
To make a long story short, they couldn't find any substance to the rumor after using an air cannon to blast the balls at both a flat steel plate and at an ordinary baseball bat. They fired the baseballs at speeds in the range 60–125 and then figured the resulting coefficient of restitution, or COR. The COR is the ratio of the outgoing to the incoming relative velocities of the two bodies. Researchers say the ball-bat COR collision is usually equal to about 0.5. They also say the kinematics involves both linear and angular momentum conservation.
Researchers also conditioned the baseballs used in this study by storing them in a 50% relative humidity environment for at least two weeks prior to the measurements.
All in all, the baseballs from the prior era had the same COR as those of today. Averaging the results from the ball-flat plate collisions at 120 mph yields nearly identical results for the two sets of balls, 0.470 +/-0.002 and 0.468 +/-0.002. Similarly, averaging the ball-bat collision at 125 mph yields 0.487 +/-0.002 and 0.491 +/-0.002 for the older and newer balls, respectively. Researchers note, however, that Major League Baseball allows its baseballs to have a relatively large COR range.
The researchers published their results recently, which you can read here: http://www.kettering.edu/physics/drussell/bats-new/Papers/CheatingPaper.pdf