In the past Youtube was unable to publish online gaming ads due to a settlement between their owners, Google, and the US Department of Justice. Even though they are still banned from publishing gaming material in the US, Youtube has announced that they will be publishing gaming material in the UK.
Ad space will be up for sale to businesses licensed to provide gaming services in Spain, Italy, France and the UK. Ladbrokes is the first company to have an ad featured but I am sure that soon, most online gaming companies will share this privilege.
As a regulatory measure, Youtube will use the same technology that they use to prevent users under 18 from watching mature-themed videos. Personally, I think this measure could also be implemented by the US government if they really wanted to make some revenue out of online gambling, but apparently they do not.