The company I mentioned above was a large multinational technology company. It c.10 years ago and I left to go into a different field of work. Even though I wasnt staying in the same industry, I decided that I did not want a confidentiality clause hanging over me for the rest of my career and didnt want any restrictions on joining a competitor in the event that things did not work out in the new job I was taking up.
To be honest, I dont think that the company was aware of the implications of the wording of my resignation letter. I handed it to my boss and she looked at it and passed it to personnel dept who put it on file. Boss and personnel staff arent lawyers and probably never read the small print in their own contracts, and as "I wish to terminate my contract on employment...." is the type of language you might expect to see in a resignation letter anyway, they didnt think anything of it. I left the company a month later on very good terms - they gave me a good written reference and even paid me a full bonus at the end of that quarter in spite of me only being there for 2 of the 3 months.
Though I did think at the time that this was a huge oversight on the companies part - this part of the contract was standard on all their contracts worldwide - 100,000s of employees. They were a company who had a lot of cutting edge technology and a large legal dept at HQ protecting their intellectual rights, patents etc.,