I don't think this is a good comparison for a lot of reasons. Microwave ovens have shielding around the entirety of the oven because they are designed to contain their energy, whereas cell phones by nature MUST project their energy. There is very little leakage from properly functioning microwave ovens. Actually, requirements state that they are not allowed to leak more than 5 mW at 5cm distance from the oven (
source). Microwaves also decrease significantly over distance and we are not pressing the microwave oven against the side of our heads.
Add to that the fact that most people spend a whole lot more time on the phone than they do standing immediately in front of an actively cooking microwave, and it's just not even in the same ballpark.
The tiny little metallic shield in between the radio and the screen is clearly ineffective for protection, which can be easily demonstrated by laying the phone down on a metal table. Still got signal? Yep. The microwaves are still hitting the side of your head when talking on the phone.
I don't think it's worth getting panicked over, but I don't think it should be dismissed either. If I spent a lot of time on the phone, I would likely spend most of it on a BT headset because it seems to me like an easy precaution to protect against something that is not entirely understood yet.