"Through EA's Ronku program, some fans are compensated for the YouTube videos they create and share about our games," an EA spokesperson told The Verge. "The program requires that participants comply with FTC guidelines and identify when content is sponsored. User-generated videos are a valuable and unique aspect of how gamers share their experiences playing the games they love, and one that EA supports."
Asked whether YouTubers openly disclose their participation in the Ronku program, the spokesperson said, "We explicitly state in the Terms & Conditions of the program that each video must comply with the FTC's Guidelines concerning Use of Testimonials and Endorsements in Advertising."
The FTC's Guidelines on Use of Testimonials and Endorsements in Advertising, in PDF form here, offer examples of personalities who would have to disclose the compensation, and those who wouldn't. In Example 7 of § 255.5, disclosure of material connections, the FTC states that a blogger reviewing a game would have to share his connection to the company if he received that product for free.
Microsoft also has a YouTube bonus program for presenting positive coverage of its products, which it calls "a typical marketing partnership."