web page do not keep a connection to the server, so there is no way for the
server to send a message to the client. the usual method is as suggested  to
have the client poll.

to trigger a client event from the server requires a constant connection.
there are three approaches for this

1) use a java applet that open a socket to the server. the server can then
notify the java applet, which can fire an event on the client.

2) same as above but use a browser plugin.

3) using a hidden frame and server push send events to the client. basically
the client requests a page in a hidden frame, and the server push javascript
to client in chucks via fluching. to do this with asp.net you need to up the
page timeouts. also every couple a minutes, the client shoudl close and
restablish the connection.

all three of these approaches cause heavy resource load on the server. if
you have too many clients, your server will run out of connections or process
threads. you will need to tune this carefully.

-- bruce (sqlwork.com)