The security firm's 2008 threat forecast said as more consumers used the iPhone to shop and bank online, malware writers would be encouraged to capture this information.
Raimund Genes, CTO at Trend Micro said users of unlocked iPhones were at most risk because none of the applications they are using would have been checked and approved by Apple.
"Users who download applications from Apple's Apps Store will be safe, but anyone downloading other software from the internet could be at risk," he said.
All consumers remain at risk from spyware and spam, said Genes, as there is no security on the iPhone to deal with these threats and no software available yet that can be installed.
"Implementing any security on the iPhones will be difficult without Apple changing the software development kit," he said.
Enterprise users of iPhones are safe, said Genes, because corporate mail servers will filter out spyware and spam before reaching the phone, but consumers who connect directly to e-mail will be vulnerable.
Consumers could reduce the risk of e-mail threats by using an internet service provider that provides filtering services, he said.
Alternatively, users could filter their e-mail themselves by downloading it through a protected computer and automatically forwarding it to a second e-mail account used exclusively for their iPhone.