Interesting Uses For Old GSM Mobile Phones?

Anonymous Coward asks: "Over the past few days, I was wondering if there is any use for old GSM mobile phones. The rate of development of these things is phenomenal, and phones seem to be becoming obsolete faster than PCs. They must have a fairly powerful processor if they are able to support real-time encoding with whatever codec (full rate, enhanced full rate etc.) they use for GSM. It seems like such a waste to throw these things away - anyone got any ideas?" How much work would it take to convert an old GSM phone into a two-way walkie-talkie?