Along similar lines, I've wondered about how often restaurant and bar owners have bugged their own premises.

There's probably a lot of valuable information discussed in such establishments.. especially, say, around Wall St.

Regarding the legal aspects, I'm not sure how much of a legal expectation of privacy one has in a "public" place such as a restaurant or bar, and we've pretty much already accepted ubiquitous video surveillance in such places (even if such videos are often without sound).

Someone could potentially infer speech just by lipreading soundless videos, but if someone did decide to use audio surveillance on their own property in a bar, restaurant, or other place of business, is there any law against that?

Quite apart from the legal and ethical objections that might arise against doing this, I'm sure some business owners are not above giving in to the temptation to spy on their patrons, especially if there's a big profit motive (like getting access to inside information by spying on Wall St execs gossiping over drinks or dinner).

That's not to mention dirt that might be revealed in conversations considered to be "private", which could be used to blackmail people or for other nefarious purposes.

The technology to perform such spying has been around for a long time, and in a bar or restaurant the owner (or rogue employee, or customer even) wouldn't have to resort to exotic techniques such as this light bulb trick. A simple microphone would suffice.