For the best example of how much investors are willing to bet on a mobile content play that can build an audience outside the wireless carriers' platforms, look no further than Thumbplay.The New York-based startup just raised another $18 million from investors including Brookside Capital Partners and Cross Creek Capital. For more details, see the company's news release in our comment section. The Series E round raises Thumbplay's total investment to a staggering $54 million, according to a company spokesman.  The company has signed licensing deals with entertainment giants such as EMI and SonyBMG, which last month agreed to give Thumbplay access to ring tones, games and music videos from top-tier artists like Christina Aguilera and Beyonce.Those agreements, along with ... [ Read more ]