FYI - I bought the Eddie Bauer leather holster w/ a magnetic closure at Best Buy and I used it for 1 day w/ the new Iphone 4. The next day whenever I pressed the turn-off button, the turn off sounds made this awful echoing noise for about 2-3 seconds. Whenever I received a message or an alert that day, same thing occurred. Didn't matter whether was using phone speaker vs. headphones - same effect. Nothing else was affected - played music / phone worked / etc, but it was discouraging none-the-less.
So I performed a system reset and the problem resolved.
For whatever reason, it appears the magnet on this holster corrupted some of the sound files. Who knows what other files it could have corrupted had I not discovered the problem early.
I'm never using a magnetic latch holster again (as appealing as they may are) and I wouldn't encourage anyone else too either.