Brod, a researcher on our Threat Research team has been tasked with tracking emerging Mac based threats. Microsoft Excel is one of the tools he uses to chart variants. From April to December 2011, there have been several dozen new Mac threats.
Well, that's nothing when compared to Windows malware — but it's definitely something when compared to the number of Mac threats seen prior to 2011.
Keep in mind that by "new", we're referring to unique variants, and not the raw number of unique binaries that we've seen. We prefer a more conservative approach when counting malware. The more generic and family based, the better.
As we correctly predicted back in May (YouTube video), Mac malware has not scaled continuously due to market share, but rather, is more the result of opportunist "bubble economies" that have produced new threats in fits and starts.
We expect more of the same for 2012.
Edit: A small revision has been to the spreadsheet linked above.