There are "growing concerns about soft errors, which are caused by alpha particles and cosmic rays that bombard the earth. Energy from these particles can cause memory bits to change status, resulting in operational error of devices...SRAM soft error rates are now actually getting worse by about an order of magnitude every generation"

"But cosmic rays are almost impossible to stop - They'll go through 5 feet of concrete without any trouble...As they pass through they can separate junction current flow for 5 ps [and cause a bit to flip]."

"A memory misfire in a PC or cell phone, applications responsible for about half of last year's SRAM sales, often goes unnoticed by a user. However, such a glitch in networking equipment, which accounted for about 24% of the 2001 SRAM market, can send information packets, such as money transfers, to the wrong address."

"IBM has discovered that SRAM tested at 10,000 feet above sea level will record SERs that are 14 times the rate tested at sea level, due to higher exposure to cosmic rays. Although alpha particles can be controlled with improved packaging, such as a plastic coating over the die, cosmic rays can never be completely blocked out...It's more or less a fact of life"

Articles on new ways to reduce the problem are at EETimes, Silicon Strategies, and EBN. Full study at IBM