On my phone, it's not just the bottom left...
When I hold the phone in my left hand in the normal manner, with the bottom left corner in my palm, signal strength drop from 5 bars to 1.
When I hold the phone in my right hand, with the bottom right corner in my palm, signal strength drops from 5 bars to 3.
When I hold the phone in 2 fingers, one on each side in the middle, signal strength drops from 5 bars to 4.
Yikes! I'm guessing Aliph and other BT headset manufacturers are quite happy to hear this, as they're about to sell a load more than they already would this time of year. Shoulda bought stock.