@OP in the article u posted they are using wifi , this can easily be done with any 2 devices that have wifi , they had to hack the phones because they are using android , else no hacking would be required , just setup an ad-hoc connection between 2 devices , and use a LAN chat client .
However , if you want to do this via GSM , there are devices deigned to do this by utilizing ISM band (like YoDude posted) , I don't know if the gsm module inside the n900 , is capable of operating at such frequencies , if it can then this can be done given an appropriate driver .