I find this very interesting. Most of my concerns with using this would be alleviated if I was able to flash my own image onto the SFP, e.g. an OpenWrt installation. (Disclaimer: no offence to the people that produce this product, I'm an equal opportunity closed source firmware avoider:)

The author mentions that 'In a more premium software package for the smart-sfp you can configure ERSPAN sessions with filters'. Selling a more expensive software package for the SFP would be a reason to lock it down and prevent others from offering competing (including open-source) software.

Another interesting aspect is the communication with the programmable logic. What is implemented in the FPGA? Is it purely signal processing? Is there packet inspection and filtering? Could the communication between the CPU and the FPGA be reverse engineered to provide a driver?

Edit: Ben, do you plan on playing around some more with these to find out if they can be hacked to run your own OS?