a) Will the new TS5 client break existing plugins? Do existing plugins need to be ported to TS5? Will it have more options functions and features than the TS3 Plugin SDK?

b) I was not able to find the Plugin SDK download in the TeamSpeak download page (Wasnt it there before?).
What happend? Why is it not there anymore? And where can I find it? Do I really need to register to download the PluginSDK?