Steve Jobs said this week that the iPhone will only run software from Apple. Third party applications won't run on the iPhone unless they've been approved by Apple.
Even ringtones and possibly other content too will need to pass Apple first before you can upload it to your iPhone. Apple says this restrictive business model is to guarantee that users don't criple their phone by uploading bad applications or content to their phone.
This should prevent the iPhone from crashing and should prevent viruses from attacking the iPhone.
Jobs says the iPhone shouldn't be seen as a mini-computer but as a new type of cell phone.