Finally had a chance to clean up the simple .NET PubSubHubbub client and callback handler I wrote last weekend. Currently implements just a Subscriber Client and an ASHX handler for processing subscription verification and push notification receipt.
I don't believe pubsubhubbub.appspot.com supports localhost callbacks, so you'll need to have a publicly exposed IP address or domain name to host the callback handler. I use DynDNS myself. (I could be wrong, I'm a dilettante when it comes to this PubSubHubbub stuff.)
NPubSubHubbub - find it here.