My 1st iRule - How can I improve this?

Is this data going over HTTP? If so, do you have an HTTP profile associated with your virtual?

The reason why I ask is that the HTTP::redirect command is only available in the HTTP_REQUEST and HTTP_REQUEST_DATA events, not the CLIENT_DATA event. If you are working at the TCP data level (CLIENT_DATA, etc), you'll need to form your own HTTP 302 response (including correct HTTP response headers/etc), that the HTTP::respond command does for you, and send it back to the client with the TCP::respond command. That's going to be a bit tricky, but it's possible.

If you do have a HTTP profile, then things are much easier for you. Something like this should work:

when HTTP_REQUEST {
  if { [TCP::local_port] == 80 } {
    HTTP::collect [HTTP::header Content-Length]
  }
}
when HTTP_REQUEST_DATA {
  if { [HTTP::payload] contains "" } {
    log local0. "Request for new XML Service Found.  Sending ASPX page."
    HTTP::redirect "http://otherdomain/otheruri"
  } else {
    log local0. "Request was for Old XML Service.  Sending to Default xml Pool."
    pool xml_pool
  }
}

Better yet, I'd assign "xml_pool" as the default pool for the virtual and you can leave the else clause out altogether.

Hope this helps...

-Joe