I had an iPhone 4 years ago but never noticed this. I have had an iPhone 3GS on T-Mobile since last fall and setting the ringer switch to silent also sets the other sounds (such as keyboard click) to off; this was handy when playing with my phone but not wanting to make any noise. However I just got a new factory unlocked iPhone 4 and setting the ringer switch to silent does not affect the other sounds. Of course this may be new to iOS 5; the 3GS was on iOS 4.