As most of us have discovered, Safari 5.1 drops support for WebKit Plugins, which unfortunately includes the very useful ClickToFlash.
Per Apple, as of 10.7 the legacy WebKit plug-in architecture is no longer supported. Going forward, WebKit plug-ins have to be converted to Netscape-style plug-ins or Safari Extensions.
Fortunately, someone has converted ClickToFlash to a Safari 5.1 extension: ClickToPlugin, with the support of the original author. This extension will actually block any plug-in, with the ability to create an exclusion list like the older version. There is also a ClickToFlash-only version available.
When I grabbed this, the only difficulty I had was in accessing the preferences. I found there was a default keyboard shortcut Option+, that brought them up. You can change the shortcut once you're in the preferences.
[crarko adds: This is a public service type hint, but ClickToFlash is too valuable a utility do do without, in my opinion. Thanks to Marc Hoyois and all others involved for providing the updated extension.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110825060344290