While not a security expert, this question does give us the chance to talk about offsite storage and shelf life issues associated with magnetic media.
There has been great debate over how long a tape will retain data. While most manufactures specify a shelf life (3490E - 15+ years, DLT - 10-30 years, etc,), independent media center National Media Labs show a 10-30 year lifetime for tape. The range is based on the chemical properties of the tape and the environment in which a tape is stored.
Another consideration, is the change in technology. Is that 3480 tape drive that you need to recover your data going to be running 30 years from now? The tape and data may survive but you also need to plan for backward compatibility.
There is a good tip submitted by Rick Cook "Encrypting Backups for Additional Security" that further explores the security issue.For additional information on the security considerations, I suggest consulting techtarget's searchSecurity.com site.
This was first published in July 2001