Kotaku have verified and published a letter sent from Valve to various CSGO skin gamling sites that basically reads like a cease and desist letter. While few of the gambling sites have made public comments, some have chosen to pause operations while trying to figure out how they can conduct business without violating Steam's user agreement. Some sites will no doubt carry on defiantly, but that could spell trouble for those who chose to gamble there as Valve will no doubt look to punish those who persist.
Below is the letter in full:
"We are aware that you are operating one of the gambling sites listed below. You are using Steam accounts to conduct this business. Your use of Steam is subject to the terms of the Steam Subscriber Agreement ("SSA"). Under the SSA Steam and Steam services are licensed for persona, non-commercial use only. Your commercial use of Steam accounts is unlicensed and in violation of the SSA. You should immediately cease and desist further use of your Steam accounts for any commercial purposes. If you fail to do this within ten (10) days Valve will pursue all available remedies including without limitation terminating your accounts."