Years ago, someone told me that a bath tub with an overflow pipe was considered "self-venting". In other words, you didn't need an additional vent pipe because the overflow pipe served the same purpose. I've looked around online, and this certainly isn't acceptable practice now.
There is a question here about converting from a bath tub to a shower. I mentioned (in a comment) that there might be venting issues, but I don't know if it's actually true.
Is this a valid concern? Or have plumbing codes (in the USA) always required venting of bath tub drain lines?