Advertising has always been outsourced by business. Typically advertising agencies are the providers and Marketing and Sales Departments are the outsourcers though frequently the CEO is deeply involved. Advertising agencies are engaged to create creative advertising for media like print, TV , outdoor, film and now web. The reasons that advertising is not created in-house is because this is a "outside" firm activity and is aimed at prospective customers. Institutionally, advertising agencies,or at least their creative departments, are free wheeling and need to be so if innovative and "new" campaigns are to be created. In-house advertising agencies are not common as companies do not see advertising as their core competence and at the same time since agencies maintain good confidentiality of "in-process" campaigns there is really not much risk of revealing your hand to competitors.
So this is a function that has always been outsourced and now is being globalized. Advertising and its components can be split up into parts that can be outsourced and this article gives a good sense of what these elements are.
Advertising agencies across the world pitch for work in the $280 billion industry. Various elements of the advertising agency work from market data analysis, creative copy writing, to cleaning up an artwork, editing an ad film, creating multiple mock ups pf packages and concepts can all be globally outsourced.
The opportunities this presents for US advertising agencies is immense. For example, if an account manager is able to get a clear idea of a project it is very possible to get large numbers of good workers working on the account "pitch" phase of the project. Quite simply if you put 10 offshore people on the job you should come up with 50 alternatives that are pdf-ed from India and you make a 5-7 alternatives presentation at the pitch stage to the client. The client gets more options and at your same level of internal people (say with a 2-4 member creative team onshore) you pitch for many more projects. Similar for analytic work.
I am unsure though as to how easy it is to actually globally outsource final creative work, or the film shoot or the media plan and media buying. Clearly these activities are best done if you are customer facing and are able to pick up what is "really" happening in your market and are able to adapt more specifically to your market situation.
This leaves US advertising and marketing professionals with the task of not only learning all elements of the advertising business to be able to manage global outsourcing but also to be able to adapt materials at short notice to customers. Clearly a lot of skills.