Does it actually prompt the user to install it? Some users are just button pressing monkeys.

Thats the idea of a Trojan. Just like the gift of a big wooden horse, the user thinks he is installing something good and gets all the nasty stuff hidden inside it.

The 'executable' files on symbian are .exe, they are inside the symbian packages (.sisx (signed) or .sis (unsigned))

Steve: "but what's wrong with a phone that, e.g., just makes calls?"

There's less profit in it!

They want you to opt for the more complicated device so you can use billable services.

Follow the money, it's all that matters.