Had one a couple of months ago. I got notified about a day or two after two pages were removed from the SERP, this was via the contact email address linked to my Google Webmaster Tools account. A message was also available in the webmaster tools account.
It was about a week after that I received the actual DMCA notice, detailing the reasons why the pages were removed. This then allowed me to file a successful counter-notification, and both pages were eventually re-instated back into the index in about 2 or 3 week's time.
It turned out that the original DMCA noticed was filed in error (or rather, it had links as part of the notice that shouldn't have been on the take-down list), and the company responsible has since apologised personally to me for the mishap.
Still, it's easy to see how competitors can abuse the system to their advantage, especially now since Google has started devaluing websites that receive too many DMCA notices.