Hi there!
First of all, I must say that I have zero experience with Arduino, or programming in general. Willing to learn though!
A friend and I are planning on sending a weather balloon to the skies. We'd like to take some photos and do some measurements on temperature, pressure, humidity and the likes.
Would it be possible to do the measuring part of this with an Arduino, and if so, which board? What sensors are available that we could use?
Thanks!
Arduino would be a good choice. Weights about 50G. Depending on your payload budget you could get even lighter with a Nano.. Example: http://arduino-direct.com/sunshop/index.php?l=product_detail&p=84 More details here: http://arduino.cc/en/Main/ArduinoBoardNano
There are LOTS of sensor choices. Some of the ones I'm familiar with are here: http://arduino-direct.com/sunshop/index.php?l=product_list&c=17 and lots here: Search Results for sensor - SparkFun Electronics
Arduino has 1KB or EEPROM where you could save data that would survive power down (or airship down??) You can add SDcard memory.
Etc.
Get started prototyping stuff and google "Arduino balloon" About 117,000 results (0.15 seconds) 
DISCLAIMER: I mentioned stuff from my own Shop...
Okay, I've done a bit more reading, and have decided I will use an Arduino. Problem is, I live in The Netherlands, so it's a little difficult to find all boards/sensors, but I guess we could order from the US and wait a while
I have some more questions though:
Would a Duemilanove do to? Is this board still supported as the Uno/Nano?
How long should I expect to be learning to use a board and program it for this purpose? I guess I'm quite computer-savvy, but like I said, no experience whatsoever...
I'm getting more and more excited for this project, just reading all these succes stories from other people who have done this!
Hi, (Disclaimer: touting my own shop)...
Quite a lot of these materials are available here: http://www.arduino-direct.com/sunshop/
The Duemilanove is the same as the UNO except for the onboard USB interface, which functions the same. No problem. And low-cost "Open source copies" AKA Clones are made from the released design files from arduino.cc .
DHL shipment from China is probably faster than from USA. Ask for quick shipment and extra effort will go into doing that.
BUT! Netherlands is Not Nice about Customs charges, and even charges Customs on Shipping cost, and they want almost-impossible documentation of low-cost shipping. (But you probably knew this...)
Get started with the simplest stuff, then "stepwise refinement"... to Get Off The Ground...