Viruses, Trojans And Worms -- Unplugged?
The biggest work in the mobile market at the moment is 3G networks - which will integrate data, voice, and video over a high-speed cellular network.
Most of the implementors are explicitly not making an assumption that the terminal is a trusted device, and are relying on the USIM to carry trusted information (such as identity information), and on encrypted network storage to carry certificates etc.
Location based services do not rely on GPS, and do not rely on trusting the phone - LBS is a network function.
Digitally signing will require an authenticating protocol (e.g. USIM->PIN) and this authenticator will unlock a cert held in a crypto vault.