Archival scientists strive to develop technology that can store media forever with out loss to the original content. This desire is evidentially justified by broadcasters, who often create historical docs using old footage, or who wish to sell on material to other broadcasters. Many archival technologies (such as tape) can decay magnetically.
In hard disk drives (HDDs), magnetic decay is usually a result of heat build up over time which increases sputtering* and this natural de-magnetisation. Although for data to corrupt through this process is rare and could take decades in a modern hard drive, it much more likely that the hard drive would outright fail before time, which prompts the need for a RAID system.
One reason modern HDDs do not de-magnetise very quickly is due to a clever process called giant magnetoresistance (GMR) which was discovered in 1988.
*Sputtering is is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. (http://en.wikipedia.org/wiki/Sputtering)
Sources:
http://en.wikipedia.org/wiki/Giant_magnetoresistance
http://www.smarthdd.com/en/bad_block.htm
http://bytes.com/topic/software-development/answers/851490-magnetic-decay-modern-hard-drives
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