This is a product a friend of mine has made and thought people here may be interested in it!
We took it on our hike of the JMT (and an ascent of Mt Langley) and it was pretty sweet....
http://code.google.com/p/backwoods-logger/
The Backwoods Logger is a programmable device for measuring and graphing temperature, air pressure, and altitude. It's designed for hikers, backpackers, climbers, skiers, trail runners, cyclists, kayakers, snowmobilers, horseback riders, and other outdoorsy people interested ...