From Gumstix User Wiki
JTAG and Overo
The Overo computer-on-module can't be "bricked", not like the older Verdex/Connex/Basix so JTAG is not needed.
Connection can always be made via the serial port. Tha ASIC ID lets the programmer automatically identify the device and start uploading code.
JTAG and Verdex Pro
Customers needing to reflash their verdex pro COM have two choices:
1/ Reflash the verdex pro COM using the JTAGpro expansion board available from Gumstix.
2/ Purchasing the reflash service from Gumstix.