I wouldn't trust the "bars" to indicate anything.
From a test right now, picking up my Blackberry 8100 (antenna along the bottom) in roughly the manner I would use to hold it to my ear, I lose about 10 to 15 dBm (from around -66 to -69 to around -79 to -81), which means the received signal power drops by around 10 to 30 times. I don't know if that's particularly good or bad; I've never had problems with dropped calls.
http://www.google.com/support/forum/p/android/thread?tid=34a...
and put your phone in field test mode
(http://gizmodo.com/gadgets/iphone/iphone-field-test-mode-let...)
An app that instructs the user to pick up the phone and hold it to his head as he would while talking, records the RSSI over a course of a couple of minutes, then graphs or displays the results might be an interesting weekend project.
On my 3GS, I can vary up to 20 dBm depending on how I'm holding it, how I rotate myself and/or the phone, where I'm holding it in relation to my body, etc.
That strongly suggests to me that this is something over and above normal loss of signal caused by picking up in a hand.
(And if the old method to access the field test mode hadn't been disabled, I'd be the first to give you dBm measurements.)