The authorities have been alerted to this incident and are investigating it. My principal concern is that the trojan was not detected by many common and popular anti-malware solutions. For that reason, I would encourage people to avail of the thread by MVP Shammoz linked to below.
[Guide] How to SCAN and SECURE your PC - Part II
This incident is an excellent reminder of why it's never a good idea to rely on one security program to protect your computer. No anti-virus software has a 100% detection rate, and the more methods you use to keep your computer secure, the better. Regular scans are also highly important.
AddOns known to have been affected;
Curse and WoWInterface have since removed the malicious versions of these AddOns, and are combing through their sites to check that no other AddOn was similarly infected. AddOn clients did not activate the trojan; it will be dormant unless you use the .lnk shortcut. If you have one, delete it.