Anyone know whats in place to stop virus's/trojans being received via websites and emails?
At the moment, it's the lack of native support for downloading apps. In other words, if it can't download anything executable, it can't receive problem software.
That said, there have been several rounds of iPhone updates due to holes in Safari that allowed executables.
The hacking community is almost always the first to identify any such vulnerabilities. Especially on the iPhone, since those same security holes were used to open the device to non-sanctioned third party apps.
In the near future, Apple will officially open the phone to apps, but they haven't revealed yet how they intend to implement security and/or testing (if any of the latter). There are several decent models to choose from, since programs have been on other phones for years.
A lot depends on trust in any case. For example, there would be almost zero chance of detecting that a disgruntled programmer inserted timed-release code into say, a Flash or Skype app that would get widely used, since no one would go through each line. (This does happen with gambling machines from time to time.)
The good thing is that if you detect a problem, the fix is often as easy as doing a restore on the device. Mobile devices are comparably easy and quick to reset to a previous version. This won't help if someone got passwords, of course, but you could change those.