So far I've seen an amazingly good response to my previous announcement, but I've also seen a lot of people with (reasonable!) questions and some thought of their own. I hope to address those here :)
"I'm A Beginner, What Does All This Actually Do?"
Basically, right now the mod community is fairly fractured - there's a huge divide between client mods and server mods, and getting these mods themselves to talk with eachother isn't so simple. We at Bukkit partnered with Mojang to introduce a new official system that should help alleviate the pressure on this, and make things easier for everyone involved. You can now efficiently talk to a client mod from the server, or vice verca, without having to worry about compatibility issues or conflicts or bandwidth or anything like that. It's all so very easy.
If you want this even simpler and a real world example of it all, think of Bukkit server plugins communicating effectively with MCP client mods. Before, this was a pain. Now, it should be very simple :)
What The System Is
The new system is a tool. It's something to use in the background and just forget about it. It's something for mods to implement silently, and it will immediately grant them 100% protection from now until the future against any conflicts on a network level. You're guaranteed that they will never interfere with the client/server if it's running something else, and that other mods will never interfere with your mod in return. You're safe in knowing that it's efficient, and you'll never waste a single byte sending data to someone who doesn't know what to do with it.
What The System Is Not
It's not a replacement ...