A natural evolution I suppose - Macs used to be the territory of pony tailed goteed *******, but they have added level-headed people to their cohorts of users thus becoming more attractive targets. Mac has become the establishment and is being attacked - just like MS.
Plus it is not a virus but a Trojan. To be honest if you apply common sense you will reduce your chance of a problem. Get a free AV programme, something like Microsoft Security Essential for a PC or Sophos for a Mac don't surf dodgy sites expecting nothing to happen (security wise), don't open executable files sent by a mate in an email that say "hay look at this", random links via ARRSE or other forums unless you can see the web address (in Chrome hover over the link and address comes up bottom left) and most importantly DO NOT PUT A USB STICK YOU FOUND ON THE PAVEMENT OUTSIDE YOUR OFFICE IN TO A DII MACHINE. This is becoming the latest way to compromise systems, send free USB sticks in the post or leave them outside offices on the floor and people will want to see whats on it. You can not legislate for stupid.
Macs had a honeymoon period but now they are becoming more popular so criminals and hackers will look at them. Plus most people now do nearly all their business online via a browser so they are becoming the main attack vectors. The old advert of I'm a PC and I'm a Mac is has been - now we do most of our "computer stuff" via a browser to the web. With Office 365 and Google Docs it makes it so much easier.
It's the opposite I am afraid. There was a very good article here How secure is an iPad? | Enterprise | Real World Computing | PC Pro by a very well known security expert. With the garden walled nature of the Apple App store it is harder for the criminals so they are looking to Android. Of course if you jailbreak or root your system then it becomes even easier (although I would hope only IT savvy people would jailbreak/root their systems and should be better able (I know...) to avoid the pitfalls).
Also I read recently that App developers are looking more to Apple for their products. Nearly all major app developers will produce a Apple app but less the same app for similar platforms. It is to do with Android OS updates not being rolled out as quickly and uniformly as say iOS or Windows Mobile.
Based on the Linux kernel, but the point is not about how easy an OS is to attack, it is about how attractive a target it is. MS used to be the 'bad boys', but I suspect that Apple is losing it's image of being 'for the geeks'. I don't think Android is really up there for target attractiveness.