Quoting wikipedia: SIM Application Toolkit (commonly referred to as STK) is a standard of the GSMto initiate actions which can be used for various value-added services.
So what are these value-added services? The most prominent would be sms encryption used by some banks to protect customer data sent over GSM. STK is supported in superold nokia bricks, as well as newest symbian/android smartphones. There's no reason why for example the n900 shouldnt support it.