That's actually impossible, at least if the iMessage is going over 3G data rather than wifi - the network handshake to accept the data connection between the phone and 3G tower would mean the text message had reached the providers servers before the iMessage was even able to start sending.
In perfect circumstances that's not going to be noticable to a user, but in less than perfect circumstances the chances are that the SMS will send considerably faster, unless your provider's SMS server is broken.
Phazer