this is my blog about my repstrap project
It might be interesting also to try building a medical tricorder. The O2 and CO2 sensors would be useful there, as well as temperature, heartbeat, respiration and the ability to pick up data from additional bluetooth sensors attached to the body for BP etc.
Wow, right after posting the above I came across an article on the Qualcomm Tricorder X-Prize competition. This link is to a draft of the Competition Guidelines. They are essentially attempting to put advanced medical scanning technology in the hands of "the rest of us". I think I will keep an eye on this as it progresses. The guidelines document can be found at: http://www.qualcommtricorderxprize.org/files/qtxp.org/QTXP_Guidelines_20120320v1.pdf