You need to define what you mean by "Instant Message" better.
Depending on what you mean, an Event Gateway may be of some use in your solution.
But a much bigger question that has to be answered first is how do you imagine users will receive communications from the server? Normal HTTP communications do not allow for the server to PUSH data to clients, no matter how they are generated by Event Gateway or not.
If you are thinking of something beyond a normal HTTP connection, such a FLAHS|FLEX|AIR client using AMF or cell phone|text clients using SMS, then it can be possible to push information to the users.
