The nature of a web service using the HTTP protocol is that
it is totally client-side initiated, so your web server does not
retain a live connection to the client in order to signal the
changes. if you really want to use a web service, you will probably
need to set up an instance of a timer class (http://livedocs.adobe.com/flex/2/langref/flash/utils/Timer.html)
in your Flex client to poll the server for a "last changed
date/time" and when the server replies with a new value, then you
should have the client pull the new data and update your client.
The down side to this approach is that the clients will only get
the update as quickly as their cycle time permits.
I believe you have an option to use remoteObject instead
which could keep an open connection between the client and server,
but I have not tried it...yet. Both of these will have scalability
issues if you have many users connected at the same time, but the
architecture necessary to handle each approach will be different,
so you should decide up front.