dk82 - 11:03 am on May 14, 2012 (gmt 0)
As a test, I changed my account from "Run ads immediately" to "Hold ads for 24 hours." The new Upcoming Ads tab appeared as it should, initially empty. I checked back in a few hours and it was populated with dozens of new ads. I switched back to "Run ads immediately" and the Shown Ads tab is again empty when I select 1/3/7 day views. Definitely looks like a bug that was introduced May 7 or 8.