Also see "What Is Perpendicular Hard Drive Technology?" in Webopedia's "Did You know..." section.

Perpendicular Recording: A Boon for Consumer Electronics
From Hitachi Global Storage Technologies - Advancements to 100-year-old Recording Technology Open Doors for 10-fold Hard Drive Capacity Expansion

Powering New Levels of Disc Drive Capacity (PDF)
Historically, areal density has increased more than 100 percent, or doubled, every year, though that rate has dropped to about 40 percent recently because of growing challenges in increasing areal densities of longitudinal recording . a technology that has been used for nearly 50 years to record information on disc drives.

TechWorld: Perpendicular Recording
The amount of data that we can store on a disk surface, in this way, is approaching its limit. As we seek to reduce the size of the charged particles we increase the risk of distortion. As a result increased areal density, the holy grail of disk manufacturing in recent years, is becoming ever hard to meet.