Yeap, thought about it. There are pros and cons between infrared and ultrasonic, but I'm pretty sure either will work. I'm not planning to do automatic landing at this stage, though, so that can wait.
there is one other thing that you may want to consider adding to your quadhelicopter robot. that is a dmu <distance measuring unit>. why ? if you set the alt to 0 and it take off one foot up then goes over a cliff you alt will read 1 while there may be hundreds of feet below or no close ground below or near. totally optional. most mobile robots have some type of switch sensor at the bottom so they do not go over stairs. like a cut off switch so when it hits or releases it flips the switch depending on how it is set up. for something flying a dmu would be better. there are several types. ultrasonic is the most common. meow. used mostly for landing.
That would be really cool.
Thanks for pointing out the Gumstix modules, cyberfish. I'm enthralled by the Overo Fire and I'm particularly interested in the package including the Chestnut 43. It looks like you could almost build your own touch screen mobile phone with that kit, except of course it doesn't include a GSM radio.
I'm an ARM junkie, and I'm excited there are things like this out there.