According to McAfee's Threat Report for the second quarter of 2011, iOS remains relatively unaffected by malware threats. Android on the other hand continues to see an increase in malware threats.

One significant change in the first quarter of 2011 was Androidís becoming the third-most targeted platform for mobile malware. This quarter the count of new Android-specific malware moved to number one, with J2ME (Java Micro Edition), coming in second while suffering only a third as many malware.

This increase in threats to such a popular platform should make us evaluate our behavior on mobile devices and the security industryís preparedness to combat this growth.

We also saw an increase in for-profit mobile malware, including simple SMS-sending Trojans and complex Trojans that use exploits to compromise smartphones. - McAfee Threat Report
Android saw a 76% increase in malware threats from the first to second quarter according to McAfee. It seems odd that Apple's iOS platform would remain unaffected by malware attempts, especially considering iOS's large portion of the mobile market share. Android may have more users in the world, but it's hardly the difference seen between Windows and Mac market share, where Apple's small portion of the market allows them a sense of security regarding malware efforts.

It is only a matter of time before iOS experiences a MacDefender like malware threat, which is something McAfee readily warns about in their report. Apple's massive market share wont be ignored forever.

Source: McAffee Threats Report: Second Quarter 2011