Just found out about this the other day. Thought it was really cool and very well done. Of course I am using this for a purpose which is not intended but it is spotting cars. Should be good. See what you guys can see ;)
Just go to http://www.maps.google.com and go to somewhere in the US like San Francisco and and zoom in. Only the roads outlined in blue are ones that use Street View so far. Then you get a popup window and you can use left right, forward and backward arrows to navigate!