Is my Mac infected with the Flashback trojan?
You can follow these instructions from F-Secure to uninstall/remove the malware:


Run the following command in Terminal: 

defaults read /Applications/Safari.app/Contents/Info LSEnvironment

Take note of the value, DYLD_INSERT_LIBRARIES
Proceed to step 8 if you got the following error message:

"The domain/default pair of (/Applications/Safari.app/Contents/Info, LSEnvironment) does not exist"

Otherwise, run the following command in Terminal: 

grep -a -o '__ldpath__[ -~]*' %path_obtained_in_step2%

Take note of the value after "__ldpath__"
Run the following commands in Terminal (first make sure there is only one entry, from step 2): 

sudo defaults delete /Applications/Safari.app/Contents/Info LSEnvironment`

sudo chmod 644 /Applications/Safari.app/Contents/Info.plist`

Delete the files obtained in steps 2 and 5
Run the following command in Terminal: 

defaults read ~/.MacOSX/environment DYLD_INSERT_LIBRARIES

Take note of the result. Your system is already clean of this variant if you got an error message similar to the following: 

"The domain/default pair of (/Users/joe/.MacOSX/environment, DYLD_INSERT_LIBRARIES) does not exist"

Otherwise, run the following command in Terminal: 

grep -a -o '__ldpath__[ -~]*' %path_obtained_in_step9%

Take note of the value after "__ldpath__"
Run the following commands in Terminal: 

defaults delete ~/.MacOSX/environment DYLD_INSERT_LIBRARIES

launchctl unsetenv DYLD_INSERT_LIBRARIES

Finally, delete the files obtained in steps 9 and 11.


Update:

Apple has released an official tool for uninstalling the malware. Read about it and download it on this Apple KB page.

F-Secure has also released a removal tool, which you can download here.