Using a Single Virtual IP to publish multiple URL's

Definitely. You don't even need to use an iRule - an HTTPClass profile would be able to do this for you. http://support.f5.com/kb/en-us/solutions/public/7000/200/sol7280.html

You would have a single virtual server and multiple pools, and then use the profile or an iRule to direct traffic to one pool or another based on the Host header.

If you wanted to use an iRule then a simple format would be;-

when HTTP_REQUEST {
    switch [HTTP::host] {
        "site1.company.com.au" {
            pool pl_site1
        }
        "site2.company.com.au" {
            pool pl_site1
        }
        default {
             Use pool attached to virtual - no action required
        }
    }
}