HA on the WSA depends on how you get traffic to the box.
If you use WCCP, WCCP will handle the HA, you just have to make sure your ACLs are setup to deny redirection for traffic FROM the WSAs, otherwise the traffic could hit both or loop...
If you use Explicit proxy you can do it via PAC file, or a load balancer... OR when the 8.5x train is available as an upgrade from 8.08, you can upgrade and use the HA feature that is in that version (IIRC, its sort of like HSRP)
(there are some security bugs fixed in 8.08 that haven't been added to the released 8.5.x train yet, you really do want to wait for them...)
