In OS X Mountain Lion (10.8) there’s a bug where the dock, exposé and cmd+tab app switching can stop showing newly opened apps. To fix this restart the dock by entering the following into Terminal:
killall -KILL Dock
I’ve had this a few times and expect it to be updated in 10.8.1.
It’s not fixed in 10.8.1, unfortunately.
I’m running 10.8.2 beta and yet to see it. We’ll see..