gaga experiment's nice, I'm planning such a thing and reading your post I was thinking that the balloon (no luck) could gets sucked into a jet/rotor engine of a flying by aircraft.
"The new layout engine in Sencha Touch 2.0 is so fast we had to use a high speed camera to measure it." source http://vimeo.com/30296006. Read "the improvement is so tiny you couldn't see it, so we slowed it down for you to see". The responsiveness on Android was horrible (check forums). Seems like the only real improvement so far.
If you just want to hook-up an API to a view it will do the job. You can check this, its supposed to be the best apps done w/ SenchaTouch so far > http://www.sencha.com/apps/
Same here. The doc provided contains very few working examples. Support on forums is nearly non-existent. They have good tutorials videos but once you start adding functionality it hits the fan. I originally wanted to save dev time but unless you're making a static app, or a RSS feed you better go with java or objective-c.
I'd add on top of that, that a bluetooth stream of data could be vulnerable to a nearby hack. If something else than the square device were to be created it would probably make use of cell network.
Actual wireless card device processor cost around $800. That let place for square to come up w/ something.
It's far stretch, but I'm trying to make learning faster and more focused than switching from tab to tab, or app to app.
Throw feedbacks at me, good or bad.