Google Street View had stopped working for me in Safari. It turned out ClickToFlash was the culprit. The ClickToFlash web plugin stops Flash objects on webpages from loading. Unless you add the pages/domains to its whitelist or temporarily allow them.
I had not realized ClickToFlash was stopping Google Street View because I did not get the usual blank Flash box where I could use the options to add a site to the whitelist or temporarily allow Flash to load for a page.
To resolve the issue, in Safari navigate to a different page that uses Flash. Doing so will place the ClickToFlash sub menu in the Safari menu. Go to the Safari Menu and select ClickToFlash » Settings....
In the Settings window that pops up, click the + (plus) button on the bottom left. A new entry field will appear at the bottom of the whitelisted sites. If you can not see it, scroll down. Double click the new entry field, and enter the top level URL (without the leading http://) of the site you want to add (e.g. maps.google.co.uk in my case).
Importantly, you MUST deselect the entry. Do this by clicking on one of the other entries in the list. Doing so will set the entry. Just closing the window without deselecting the new entry will not set it.
[crarko adds: I didn't experience the problem with the U.S. Google Street View site. Perhaps they are feeding alternate H.264 video here.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20101011092829655