mobile phones are not receiving spam targeted at them or seeing the same barage of advertisements that a desktop user sees.
the spam thing may change (especially if there is a way to identify vunerable users), but the number of portable users is much less than the number if normal computer users, and the connectivity of the portable users is significantly less (although it may be on full-time). as a result taking over thse system is less attractive to the bad guys.
the smaller screen means that a 'typical web page' with a paragraph of info surrounded by advertisements is unreadable, so users don't go there, or if they do, would have to go to extra effort to see the advertisements.
so, for all these reasons, I just don't see the mobile malware problem ever getting as bad as the windows malware problem currently is.