For the last decade, viral videos have pushed the envelope of what constitutes an ad. Remember the infamous sock puppet ad for the now-defunct Pets.com that ran during the SuperBowl? That was long before YouTube, and just one early example of strangely compelling marketing exploits.
Another trend, one that has been around for at least a few years, is to make an ad look like pure entertainment. The recent VW ad where a kid is acting like Darth Vadar is one good example, and many of the recent T-Mobile ads show a magenta-clad biker racing around without ever using a phone. The casual viewer might not even realize which company is involved until the logo appears.