While I am on the subject, I am often stunned and amazed at the lack of a built-in ND filter on today's cameras. Not only is it easy to implement, it would be another marketing feature manufacturers could use to differentiate their products and favourably compare with video cameras.
I don't think it's as easy as all that. You're introducing another element of glass between the shutter and the bayonet mount, and you'd need an assembly the can rotate it in and out. With a fixed lens camera, that's not so difficult to design, but once the lenses are interchangeable, it becomes more complicated.
E-M5 | E-PM2 | mZD 12-40/2.8 | mZD 12-50/3.5-6.3 | P 14-42/3.5-5.6 II | P 45-150/4.0-5.6 | P 14/2.5 | P 20/1.7 | PL 25/1.4 | mZD 45/1.8