Helpful Links Regarding Flashback Trojan
A link to a great User Tip about the trojan: Flashback Trojan User Tip
A related link in the tip to a checker: Malware Checker Dowload Link
Another excellent reference on malware: Mac Malware Guide
A Google search can reveal a variety of alternatives on how the remove the trojan should your computer get infected. This can get you started.
For now I recommend the User Tip from etressoft to detect and remove or one of the following:
Kaspersky Flashback Trojan Site:Flashback Trojan Detection and Removal
F-Secure Tool: Flashback Removal Tool
Also see Apple's article About Flashback malware.
Go to F_Secure's Flashback Removal Tool web page, download their Flashback trojan detection/removal tool, and follow the instructions you find there. In my opinion this is currently the most complete and reliable tool available.
Running Software Update (found on the Apple menu) should get users of OS 10.6 or 10.7 the appropriate update(s) for their systems. Note that you will not see (or need) the update if you run OS 10.7 & have not installed Java.
You should always try to keep your Apple software as up-to-date as possible. Apple frequently releases updates that eliminate vulnerabilities attackers might try to exploit. Some security firms estimate as much as 80% of all successful malware infections occur on computers running out-of-date software after an update has been released that would have prevented that.