LR3 includes a new export interface, the Publishing Manager. Essentially, it allows for more export automation, using monitored folders. With a third-party plug-in--it only ships with one for Flickr--it will allow you to automatically synchronize exported photos and download comments. Without a real plug-in, it's just a bit more powerful than the existing export tools, because it tracks which photos have been uploaded already.
However, it feels slightly undercooked. That's partly because there aren't yet any third-party plug-ins for sites like Facebook or Smugmug. But it's also frustratingly disconnected from Export. Even though most of the settings are the same, you can't import or link to existing Export settings when creating a new service, and if you decide you'd rather use Export--or need just a slight variation of an existing service for ad hoc export--you have to recreate it from scratch. It's also not integrated well enough with the Smart Collections; you should be able to designate collections to monitor within the service, for rule-based automatic publishing.