Thanks I'll definitely take a look at GainSpan. Yeah I don't necessarily want to build everything from the ground up if the components have already been built.

Here's an example though. Lets take the fitbit as a very basic example. Mainly it's a well written piece software that needs to interact with multiple sensors: gps, gyroscope, heart rate monitor, probably temperature.

Now, I can grab a gyro and an arduino project to read and utilize data from that gyro, as well as all the other components... Separately. But I'm not sure where to go from there in order to turn all that into a single embedded system. It's possible I'm just repeating myself and you've already answered my question, lol. Maybe I just haven't gotten my hands dirty enough yet.