This must be a misunderstanding. Nokia has not open-sourced their browser. They have open-sourced a port of WebKit to their own proprietary S60 platform. The usefulness of this for other developers is limited. The UI of their browser is closed source and Nokia -- being a strong lobbyist for software patents in Europa -- are probably patenting it. Here is a quate from an article that at least asks some critical questions:

Opera Software's chief technology officer, however, doesn't expect Nokia's move to have much of an impact. "It has limited value for the open source community," said Hakon Lie, CTO for Opera. The amount of code that Nokia released is relatively small compared to the amount it kept proprietary and the innovations Nokia has made are unlikely to be useful to developers of mobile phone platforms other than S60, he said.

"What I'm seeing is they're flirting with open source and trying to get the open source community interested in their platform, but it's more of a marketing thing rather than a real technical contribution," he said.