Thanks for posting this update. Glad that it's working for you!
To confirm: with the 4.2 beta firmware, it is critical to erase your Netduino and reflash both the bootloader (using SAM-BA) and the NETMF runtime (using MFDeploy). Otherwise, they'll have the flash layout mixed up (leading to what you just saw).
Chris
This wasn't in the original step by step flash instructions, caused quite the headache!