However you have also made a convenient assumption that it will utilise two usb ports to power it, allowing it to have twice as much power, for a max of 5 watts. That is abit of a stretch to assume that and why the math to me does not add up for it to do 3.5 Gh/s at 2.5W and is what I stated.
See Lethos, I was right. I correctly predicted 3 months ago that it would use 2 USB ports, and BFL confirmed it:
The Jalapeno draws it's power from two USB port connectors. Both need to be connected to achieve the full 4.5 GH/s performance. (At the users discretion, a single USB connector can be used if it has a 2nd head split off as is common for use with laptop DVD drives).