Answering a few questions here...
(1) the CO2 bubble impact was a very nontrivial part of the design. So far this looks like it's been addressed very well.
(2) the receiver vs. direct bluetooth from the sensor is a good point. Our first version has the BeerBug sensor "buoy" with an onboard infrared radio. The receiver box resides outside the vessel, but still in the vicinity. It reads from the infrared sensor, displays and stores the data, and (optionally) provides a bluetooth link to a mobile device. So bluetooth comes from the "box" rather than the sensor. I''m not sure how to attach pictures here, so instead here's a link to the picture of the BeerBug (left) with the receiver display (right): http://tinyurl.com/cf6tk4a
As was mentioned, we could put bluetooth right on the BeerBug and skip the receiver box altogether. So the cost for a standalone BeerBug SG sensor would go up slightly because of the bluetooth module addition, but the need for the receiver/display would be eliminated and the overall product cost would be lower by $30-$35 or so. The reason we haven't decided to do this for original product is because we're still collecting info on the home brewers' interest/willingness in using a mobile device (or other bluetooth-enabled device such as a PC) to collect data.
The expectation was that everyone could use the local receiver/display and a subset would want to incorporate using their mobile device. The local receiver/display provides the ability to do both, but yes it does cost more to have two separate modules instead of one integrated module. If there's strong interest in reading directly from your mobile device (Android/iPhone/iPAD), we could do a spin on an integrated BeerBug without the local receiver/display.
Can't say this often enough, thank you for the great info!