MSNBC.com will test an ad-subsidized news service for cell phones in a bid to gain a wider audience. U.S. wireless service providers now put everything from Web surfing to video and music on cellular phones. Media firms have been eyeing this trend as a potential opening for using the mobile phone to expand ad revenue and viewership. MSNBC.com, a joint venture of Microsoft and NBC Universal News, is one of the first U.S. media companies to test the theory with a three-month experiment that it hopes to expand into a full-on service by the end of summer.