Smartcard and other such hardware tokens (including crypto accelerator boards) and the appropriate software interfaces to such devices.
All of the following are details about RSA capable Smartcards.
One of the more interesting aspects of GSM is the use of Smartcards for access control and storage of phone lists and messages.
General background information:
General programming information
Hardware connectors
General background information
http://www.intertrader.com/library/DigitalMoneyOnline/dmo/dmo_c.htm
GSM has a messaging interface called SMS (Short Messaging Service) which allows 162 byte messages to be sent to the mobile phone. This is what is normally used for notification of the availability of voice mail messages but can be used as a fairly generic paging interface.
Getting details from your local mobile phone network company is almost universally challenging.
There are a number of gateways available online at the moment which offer free email to SMS gateways. There are also a few commercial services which charge registered users for traffic through the gateway.