If your DNS all points to the same VIP and your backend servers know which content to serve up from the host header, you don't need a rule to handle this. BIG-IP will pass the headers and original request unmodified by default.
However, this is only true if the sites are not SSL, because there is no way to have multiple SSL certs attached to the same IP (wildcards aside). Further, an iRule won't help because you have to decrypt before the iRule processes, and to decrypt you have to use a cert. So if SSL is involved, you will need to use separate IP's.
Denny