The idea is a consumer product that is also hackable...thus incorporating a full arduino instead of just a pic. I want it to be simple enough for people to use, yet open source enough for people to hack.
If I were to dump data over the serial port, I'd have to write an application for a computer. For example, Suunto watches require a special software app to download data, Garmin GPS units let you just browse to the log file. Suunto is lame, Garmin is user-friendly. I'd rather be in the latter category.
So, I'm looking to log data, plug into the arduino's USB port, see it recognized as a USB drive, and browse to the file. It might not be possible while maintaining the ability to program the Arduino over USB. It might just be easier to have two USB ports.