Lots of people have done this sort of thing – look up JP Aerospace for one. Also there is a Cambridge UK team did one recently. None use real time transmit back to ground – wifi doesn"t have the range. Might be able to get 3G working though, but even that is range limited. Satellite phone?
Completely not true I"m afraid! Happily most of the ones launched in the UK have real time transmissions to the ground using small FM radio modules and amateur radio protocols. There have been over 80 high altitude balloon launches out of Churchill College in Cambridge since 2006. People with receivers can track them and upload the telemetry in real time, which then gets plotted on a map online. More info in the UKHAS link below.
Using the raspberry pi, just have it feed radio teletype (lots of linux ham radio software can produce it) out of the audio jack and into a radio. Check the legality of flying radios in your country. There are restrictions on frequency and power in the UK, for example.
The UK High Altitude Society (UKHAS) has lots of info on how to do it, more importantly how to do it to maximise the chance of it being successful and safe.
The Cambridge group is Cambridge University Spaceflight (CUSF) and they host a number of launches for their stuff and for others who want to come and use the launch site which has all the necessary aviation authority clearances and permissions.
It"s good fun, and an excellent way to get a ~1kg experimental payload to >120,000ft. You"d have to be a lottery winner to do that with rockets. Rasp pi is 1000 times more that you need computationally to log some sensors and take photos, with a power consumption that reflects that, so do your sums on batteries carefully – remember their performance is rarely as good at the low temps of high altitude. But, with all that computational power to play with, just think what awesome things you could do!
(one-time president of CUSF)