So, here is the basic layout of my tracking system:
(The top section is part of the balloon payload, and 
bottom is on the ground with me :)


GPS+GSM         MICRO             
 -----          -----               RADIO TX              
|     | RS232  |     | SIM. RTTY  -------------   |
|Telit|--------| PIC |-----------| Radiometrix |  |
|GE863|--------| 18F |-----------|    NTX2     |--| 434.6
|     |        |     |            -------------     MHz
 -----         |     |                              RTTY
               |     |
               |     |
               |  ADC|---|Baro External
               |  ADC|---|Temp Internal
               |  ADC|---|Temp External
     Buzzer|---|     |
                -----


  CELL                                       AOR8200
 PHONE               NETBOOK                  RADIO   |
 ------        --------------------           -----   |
|      |      |     dl-fldigi      |         |     |--|
| SMS: |      |                    |---------|434.6|
| GPS  |      |  decoding 50 BAUD  |  AUDIO  | MHz |
| POS. |      |    RTTY string     |         |     |
|      |      |                    |         | USB |
 ------       |  TIME,POS,ALT,FIX  |         |     |
              |                    |         |     |
               --------------------           -----
             / .................. /
            / .................. /
           / .................. /
          /                    /
          ---------------------


The main components:
1. GPS+GSM Module    (Telit GE863)
2. Microcontroller   (PIC18F2520)
3. Radio transmitter (NTX2)
4. Radio receiver    (AOR8200)
5. Netbook running dl-fldigi
6. Cell phone

Not shown (not part of the tracking):
1. Digital camera    (Canon A470 running CHDK)
2. HD Video camera   (GoPro HD Hero)