How so? I've only heard of one instance of a trojan compromising a smartphone, yet I've heard millions of instances of trojans, virus' and other malware, spyware, adware infecting computers and costing people thousands of dollars.
So how do we have less control on a smartphone? All you need to do is format the system, which IIRC the iPhone actually has and can be done remotely (like any iPod can) through iTunes. Similarly the Pre and any phone can have its data storage 'formatted' and be reset to factory defaults.
Aside from taking my computer back to the medieval ages (IE full format) I have little beyond anti-virus programs that truly protect my OS from anything. Every phone's OS is on read-only memory that can be reset at any time by the push of a button. IMO that gives me a lot more control over my phone than my desktop.