[derefr]

As an alternative, rather than requiring that all Flash shims have some sort of screen real-estate of their own to overlay a click-to-play control onto, perhaps hidden embedded Flash (or Java, or ActiveX, or whatever) objects could request the browser to prompt for their activation with an information bar.

[cheald]

Chrome does it somewhat unobtrusively by sticking an icon in the URL bar: http://cl.ly/3I1q3k0X0R1I2G2X1p1S

Works great.