Not only are they loaded with all sorts of personal and possibly business information a crook would like to steal, but most smartphones are also completely unprotected. And most people are not aware of the threat.
"We've definitely got to start to worry about security on mobile devices," said James Lyne, an expert on mobile security at Sophos Labs, one of the giants in the data security business.
"For the last few years, it's really been more of a hyped topic. But over 2011 we started to see the bad guys produce some nasties just like on the PC for these mobile devices. So it's more important we're protecting ourselves," Lyne said.
All smartphones and tablets, including the iPhone and iPad, can have security vulnerabilities.
But right now, most mobile malware is aimed at Android devices.
That's because Android powers more smartphones, and it's an open platform, which makes it's easier for the bad guys to distribute their malicious software.
So what are the threats?
"The things they are doing on PCs, they're also doing on smartphones - and even more," said Gary Davis with McAfee.
Davis says if cyberthieves can get their software on to your smartphone they can spy on you, run up your wireless bill or steal your personal information.
Take banking Trojans as one example.
"They would intercept your financial transactions that you're doing with your bank and then use that to drain your bank account once you've had access," said Davis.
Spyware can harvest information about the places you go and when. It can also record phone conversations and forward them to the attacker.
And of course, you can always click on a malicious link yourself or be tricked into giving out your personal information via a phishing scam directed to your cell phone.
What can you do to protect yourself?
The first security software for smartphones is now available and more will soon hit the market.
But do you really need security software for your mobile devices?
I put that question to Paul Reynolds, electronics editor at Consumer Reports.
"We don't think that people have to install yet another program for security on their phones, at least not now. Probably the biggest security threat is losing your phone," said Reynolds.
Lyne agrees. He says mobile security today is about the basics: have a decent password, use encryption and make sure your device is patched - running the latest versions of both apps and the phone operating system.
But he says in the next year to 18 months, you probably will need to seriously consider security software, especially if you use your smartphone for shopping or banking.
You also need to be careful about the apps you install. Think before you download. Check reviews. Be skeptical.
If you go to Amazon or the iTunes store, your chances of getting malware are relatively low, but still possible. You run a greater risk at the Android Market. The risk goes up dramatically for Android users who get an app from an unknown website.
Smartphone hacking will rise in 2012
McAfee: Top Five Tips to Avoid Bad Apps
Sophos: Mobile Security Toolkit
Sophos: 7 Tips for Securing Mobile Worker
Best Practices for Safe Mobile Device Usage