select member based on cookie in request

I think this is a start to what you are wanting to do. I would suggest looking at the HTTP::cookie Wiki entry to see what else you can do with it and then maybe searching DevCentral for more uses of HTTP::cookie.

Perhaps some of the others know of a better / different way:

This iRule should compare looking for two different values of the cookie. If neither match then the traffic should proceed to the Pool assigned to the Virtual Server.

 when HTTP_REQUEST {
if { [HTTP::cookie value "some.cookie.value"] } {
node 192.168.1.1 80
}
elseif { [HTTP::cookie value "some.different.cookie.value"] } {
node 192.168.1.2 80
}
}

HTTP::cookie Wiki Entry:

http://devcentral.f5.com/wiki/default.aspx/iRules/http__cookie.html

node Wiki Entry:

http://devcentral.f5.com/Wiki/default.aspx/iRules/node.html