I noticed about half the time it doesn't vibrate for me either. The screen doesn't turn on or anything. I just wake it from sleep and it shows me a text that's been there for a couple of minutes. Occasionally I look at my phone and I have a missed call and I totally didn't feel my phone vibrate.
A buddy and I made a trek last week to the nearest Apple Store. His 2 year old Macbook Pro's battery had literally swelled to the point where it had separated the main case and it would launch and shoot out when he unlocked it from the chassis.
The Apple tech told him "Oh, that's pretty normal, but we'll replace it for free anyway". I was standing a few feet away playing on an iMac and I almost fell over from shock that the guy would actually say that was normal.