Malware monitoring site F-Secure and Twitch.tv are today both warning viewers of gaming streams that new malware is abroad which seeks to empty your Steam Wallet and inventory. At present it appears that you can only become infected through risky behaviour - falling for a phishing attempt - but the threat is significant enough that both are making users aware of the dangers.
F-Secure report that a Twitch Chat bot spams links to 'CSGOPrize' which offer Counter Strike Global Offensive items through a lottery system. When filling in the web-form the malware hosting site downloads software to your PC giving the account hijackers access to your Steam Wallet and items, which are then offloaded quickly and without your knowledge. As the process happens on your PC it doesn't trigger the normal protections Steam has in place warning of an external intrusion into your account.
At present there doesn't appear to be a cleanup procedure if you're infected, so following through with sensible browsing habits - only clicking on links which appear legitimate, running a script blocker etc. - are you current best protections.
More information can be found at F-secure.com.