I'm guessing an external antenna. Most phones have these on the back somewhere. Don't have my Incredible yet (just ordered it today, how will I survive?!?) but that would be my guess.
The connector on the back of the phone is an "FME" style connector.
Here's a link that explains all the various cell phone antenna connectors:
Connectors used in the cellular and PCS Industry
Here's a few links to places to get these antenna's.... I use one when I go hiking which allows me to send and receive texts when I get towards the tops of mountains:
Cell Phone Antennas and Cell Phone Boosters at Wilson Electronics
Buy.com - Wilson Magnet Mount Cellular Antenna
Wilson Mini Magnet Mount Cellular Antenna - FME Female (301113)
wpsantennas.com - The ARC Wireless Freedom Antenna (FR0803R30-FME) Boosts Your Cell Signal and Improves Your Wireless Performance
Wilson 301146 Cell Phone Cradle with Built-in Antenna (FME) (301146) - Wilson Electronics - 301146 -
I hope this helps!