Crunchgear has the details on what the 2.0.2 fix did for the 3G iPhone, as discussed by AT&T.

In summary: the iPhone was requesting too much power from the cell tower, causing the cell tower to terminate the connection! The 2.0.2 fix reduces the power requested by the iPhone.

If you have had problems or have been following the general topic, i recommend reading the full comment here: http://www.crunchgear.com/2008/08/28/straight-from-att-were-working-on-reception-problems-but-its-your-fault/.