Google Wave is now fully available to the public, but still in BETA stages.
You no longer will need an invitation to sign in to Google Wave, just use your Google Account.
Google Apps for Businesses can also turn on Google Wave for employee accounts.
The release of Google Wave to the public is one step towards Gmail being replaced with Google Wave… personally, unless they really overhaul the current Wave interface and it’s bugs, I am not looking forward to this, but hopefully the changes in the next few months (or, possibly even years) will cause Wave to be more desirable.