>> The tricky bit on these systems is how much effort/time it takes
>> to move data and context between the "main" CPU and the FPGA and then back again
FGPAs are much more efficient for signal processing from data collected from an A/D converter (e.g. software defined radio) or initial video processing from an image sensor.
Netezza does this with their hardware, I've been dreaming of the day I could DIY and put a OSS RDBMS on top.