We tried something like this with an RSS feed on ONE of the forums (
Java in General, i believe).
The additional load on our server brought it to its knees, so it was quickly shut down.
I would imagine that any standalone app that ever so often pinged the server for updates would cause a similar increase in workload, and also kill the server.
I'm sure someone will correct me if i'm wrong, but you can search around this forum for more details on that.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors