Us Mac users have been spoiled.
So what to do? Well, if you trust me, download this script:
... double-click the .zip file to decompress the .app, then double-click the .app file. It's a simple Applescript that checks for the 5 evidences of Flashback that F-secure, Mashable, and cnet contributors have found. It'll pop-up a dialog telling you whether you need to worry or not.
If you're infected (or if you don't trust me =) ), that cnet link can help you through it yourself.
Apple already has a fix, so if you're clean, just update your system if you haven't lately.
You might be tempted to think this marks the end of virus-free Macs -- and it might be, but I doubt it.
* UPDATED with info from Macworld story, apparently it installs just by visiting a malicious website.