Client request web page from server. Clent then requests for extra calculations to be done; server performs series of calculations and sends partial results as soon as they
I would take a look at orbited
They use several comet transport implementation that they choose based on configuration and browser sniffing.
See http://orbited.org/svn/orbited/trunk/daemon/orbited/static/Orbited.js
and look for "Orbited.CometTransports"
Some of the different transports must be matched by the backend implementation, so have a look at the server side for orbited also.