My stupid vibrate switch on my 3GS is acting exactly how banderson6793 described his functioning: you wiggle the switch when it is in the vibrate position (sometimes, even so much as simply breathe on it), and it goes in and out of silent mode. This happened after about 5-6 months of ownership, in my case, and no abuse.
Although I'm confident Apple would replace my phone without blinking, I am loathe to get it replaced by Apple because when I first got my phone, I had to suffer through 2 crappy LCDs before I got a good one, and in addition to that, I not only currently have a 3GS that doesn't require a computer tether to jailbreak, but which I also have the SHSH blobs for a 3.0.1 downgrade for.
Other than the switch, my phone is in perfect cosmetic condition, thanks to it living in a BestSkinsEver since day 1. I'm waiting and hoping that I can eventually discover a way to fix the switch on my own, or find somebody who can fix it for me. In the meantime, I've been using a software toggle to turn silent/vibrate mode on and off (SBSettings toggle), which works alright.
On a side note, I've become increasingly frustrated with the quality of Apple hardware. Love the software, but the hardware quality control is "meh." If I eventually choose to get this phone replaced for this issue, it will have been my 7th phone. (1st 3G: defective dock connector port; 2nd 3G: defective accelerometer; 3rd 3G: mute switch falls off while toggling it (!); 1st 3GS: washed-out LCD; 2nd 3GS: washed out LCD; now 3rd 3GS: defective mute/silent switch) Also, a recent iPod Touch purchase ended up having a defective power/sleep/wake button (pressing it did nothing). I "consume" a lot of consumer electronics, and I have never, ever had this kind of poor luck before with any other make of hardware.
-- Nathan