Just received the invitation from Google Music. Below is a screenshot.. Currently they support streaming over web as well as an App for Android. For iOS or iPhone folks the alternative way is to use the "safari" browser just like with Amazon.. (More details here at Cnet). Actually I am not a great fan of Apps and Apps effectively kill the development of full browsers on mobile phones. Still I hope there comes a day when we can simply access the stuff on mobiles just like a normal, regular browser.
Looks good... Currently they are offering only in USA. But as usual we all know how to be in US to activate accounts :)