I tried to do it myself and you pretty much have to write your own textarea implementation in Javascript. Wave as an idea is good. As a protocol I think is horrible for Javascript because either implementing the straight server-to-server spec is impossible on a regular textarea, or you need a beefy enough server to parse full-text states between keys for X many users.

While Wave is a good idea, and the hype is justified for a closed system, implementing it on your own system requires a deep, DEEP knowledge of Javascript.