since yours is only two days old, it may be the "corrected"

http://www.fudzilla.com/home/news/latest/iphone-4-silent-recall-may-already-be-in-effect

Pretty much it seems that apple is now applying a microlayer of nonconductive material around the antenna. Look at this picture in the second link. If you hold the phone in a certain way where your hand is bridging the 1st antenna with the 2nd antenna your hand will be a conductive surface and detun the antenna slightly. This will reduce your signal by 24 db. Now applying a non conductive surface on the antenna such as a microlayer of plastic, tape, or a case will reduce your signal slightly but not 24 db.

http://www.anandtech.com/show/3794/the-iphone-4-review/2

The old way AT&T was measuring bars is as follows

5 bars 91 to 51 (lower is better)
4 bars 101 to 91
3 bars 103 to 101
2 bars 107 to 113
1 bar 113 to 107

The new way AT&T was measuring bars


5 bars 76 to 51 (lower is better)
4 bars 87 to 76
3 bars 98 to 87
2 bars 107 to 98
1 bar 121 to 107

107 is where you start having trouble
113 causes you to drop calls.

Thus if you had a signal strength of 89 (5 bars with the old metric) hold the phone in the correct way with no nonconductive materials breaking your conductive hand you could drop your signal to 113 and now get drop calls. From 5 bars to 0 bars instantly based on how you hold it.

Now a days your phone may have the same signal at 89 db before you hold it, but now it is reporting 3 bars and if you hold it the wrong way it would be 1 bar. Furthermore now a days it appears at&t is using a microlayer of something to make the metal less conductive when you hold it.