Implementation ideas for software defined radios
One way is to use for this purpose cheap USB TV tuner. I've read that ppl have good results with it. Here is the link:

http://hackaday.com/2012/03/30/working-software-defined-radio-with-a-tv-tuner-card/

Obviously it is viable only for receiving (so no, it is not possible to make GSM base station with it). 

However GPS receiver is doable:

http://hackaday.com/2012/04/13/those-usb-tv-tuners-used-for-sdr-can-also-grab-gps-data/