Bukkit has announced a new approach to Minecraft server and client mods that has been developed by Mojang and Bukkit. It is very simply a way that servers and clients can communicate with each other so as to minimise the chances of the communications being a load of game-crashing gibberish.
Bukkit's Dinnerbone has described it on his own blog:
What am I talking about? Well, imagine that the server knows there is a client mod that can give players red hats, but the players need a client mod to support it. Player Bob has the client mod so his client can happily be told to put red hats on players. Player Isaac though doesn't have the client mod, but he keep getting these strange messages from the server that crash his game. The new tool is to prevent this from happening.
How does it work? Everytime a player logins, the server and client tell each other what mods they have. If a mod-related message is to be sent from one to the other, it isn't sent if it is known that other end won't understand the message, i.e. the required mod isn't installed. Therefore, Player Isaac will never be sent the hat order because he doesn't have the required client mod. However, the hat order will still be sent to Player Bob.
What does this mean for the game? Well, it does not mean that Bukkit will start making client mods, but it will (hopefully) mean that client and server mods will no longer interfere with each other as it will be clear where every signal is coming from, e.g. the hat order comes from a hat server mod, not a enchantment server mod. It is hoped this means that future client and server mods can be more harmonious. This is not specifically for Bukkit and their mods and plugins.
Hope that it understandable as I have tried to just explain the basics. See the links above if you want more details.