insecure memory warning

[hardpack]

i'm a new user of gpg, and i've installed it to encrypt data files,
not for delivery, but for secure storage.

i'm running red hat linux 5.1 and have installed gpg from the source
(not from any rpm).

since i'm encrypting for secure storage (and not for mail), i've 
chosen to use the --symmetric option with gpg.  when i run it on
a file, i receive this message:

  gpg: Warning: using insecure memory!

what does this mean?  how bad is this? 

also, is --symmetric the correct option to use?  and how strong is
the encryption when there is no passing of keys?  

thank you.