N900 Hardware Phone
The N900, unlike previous Nokia internet tablets has a phone module.
Hardware
The phone is a 2G/3G modem, with quadband GSM, and several bands of 3G. It may not have the required band for 3G in your country, for example, some US and all canadian carriers will support EDGE speeds at best.
The phone module controls the GPS, and contains software to generate locations.
Software
For the GPS, liblocation is used.
The modem can be accessed 'directly' - through pnatd.
This thread on the maemo-developer mailing list gives some examples of using pnatd - but it is not a reference guide.
ICD is the daemon that deals with internet connectivity, including managing data connections to the phone.
More documentation on ICD is found here.
SMSs are stored using eventlogger
wappushd-dev