We ordered a Barometric Pressure Sensor (BMP085) from Sparkfun, And the Protoshield (piggyback prototyping board) from Hacktronics is on it’s way. We have a software library that enables us to run two I2C data buses so we can communicate with the two SHT21 temp/humidity sensors (same I2C address) from MisensO.
http://todbot.com/blog/2010/09/25/softi2cmaster-add-i2c-to-any-arduino-pins/
Discussion concerning how to use this chip can be found at http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1280074353 and the library with examples at https://code.google.com/p/arducopter/source/browse/trunk/libraries/#libraries/APM_BMP085