On April 1st, April Fools day, Google released 3D Street Views and it stayed up for about a week after the 1st. Then suddenly the feature dropped off the map and it was gone.
Users started to complain and Google has heard their voices.
Alvin from the Google Maps team updated a Google Maps Help said:
Thanks for everyone's thoughts in this thread. I passed on all the feedback to the Street View engineering team. They know how much everyone wants the feature back (I want to use this also!), so hopefully Street View 3D will make its return soon.
3D Google Maps street views may indeed return!
Forum discussion at Google Maps Help.
Update: On April 21, 2010 it seems Google brought back 3D street views using a different form of activating it.