The most recent announcement at today’s Google I/O in San Francisco is that Google Wave is available to anyone who wants to use it. http://wave.google.com will take anyone to the sign-in page and voila, wave away.
From personal experience, Wave has some hurdles to overcome in order to go mainstream. The same organizational mindsets one has to adopt when using gmail are the same when using Wave: threads can get crazy out of hand quickly. The knowledge needed to make both Gmail and Wave usable isn’t necessarily an intuitive thing – there’s some digging to be done.
That said, the recent improvements to Wave have made it much easier to manage and a lot easier to use. Let’s see if “Generally Available” makes a difference with regards to usage.