IRC servers can have lots of clients that receive messages, and they have no problems sending thousands of messages to connected clients. Diaspora's nodes seems to be both server and client, but the mechanics are the same. When it receives a message, it's just like a IRC client, and when it sends a message it is like an IRC server.

Unless each physical server is overloaded with busy nodes, I think it will scale well. There is a limit to how many friends that subscribe to an average user.

A possible improvement could be to have "repeater" nodes for popular users with tens of thousands of readers. Instead of sending 20.000 messages, you only send messages to 2-3 repeaters for example. The subscribers would be connected to the repeater node, and not the original node. I imagine that this could be controlled by setting a privacy level on your messages. Public messages would go to the repeaters, while private messages would be sent directly to your closest friends and family

OTOH, if you have 20,000 "friends" maybe Diaspora should force you to buy a more powerful server. Social networking is one thing and social marketing is another.