There are a bit less than 700k that are plugged in to electricity and connected to the internet. Of those, a bit less than 500k are actually used by anyone, that is out of around 8 million devices we have currently in operation, so you are targeting, relatively speaking, a very small number of devices if you target 3.1 firmware, although 500k families is nothing to sneeze at.
Edit: figures above are for USA only. The only unique other firmware is for Sky boxes, and that is similar to the 4.9 firmware we had a year or so ago.
- Joel