The iSeries has security built into the hardware instructions. The machine
instructions will not allow access to data unless the user is authorized.
Applications implemented using Intel instructions must add additional code
to validate user access as part of the application. Since the application
is responsible for security there is an increased possibility of the
application designer missing or incorrectly checking user access in the
Intel based applications. While in the iSeries programs the hardware
enforces access control and cannot be circumvented.
 
Virus and hacker protection is an integral part of iSeries' design and
architecture; most common viruses written today simply cannot 'execute'
and propagate in the iSeries environment.

IBM says:
- iSeries (AS/400) delivers the security offerings that provide you the
  confidence you need to simplify and grow your e-business securely.
- iseries (AS/400) leverages both its strong base system security and
  industry security standards, as it extends security to Internet operations.

See also:
http://www-1.ibm.com/servers/eserver/iseries/whpapr/internet_security.html