iRule Redirect Loop

Try this:

when HTTP_REQUEST {
if {([class match [string tolower [HTTP::header User-Agent]] contains "mobiledevices"]) && !([string tolower [HTTP::uri]] starts_with "/mobile")} {
  HTTP::redirect "http://[HTTP::host]/mobile"}
}

The && !() statement is the important one here. You're basically saying only redirect to /mobile if the inbound user is a mobile device that isn't already on the mobile portion of the site.

Colin