For a short period, Nokia put the smartphone operating system in the hands of the Symbian Foundation and opened up its source code, allowing other device vendors to modify it as they developed their own products. But Nokia recently took Symbian back in-house, and last week the company acknowledged that the source code is no longer openly available.

An official March 31 blog entry about caused some confusion because it was entitled, "We are Open!" Critics said under its new licensing scheme, the OS was no longer "open" in the sense of being open source. In another blog entry last week, Nokia ."

"As we have consistently said, Nokia is making the Symbian platform available under an alternative, open and direct model, to enable us to continue working with the remaining Japanese OEMs and the relatively small community of platform development collaborators we are already working with," the April 4 unsigned blog entry read. "Through these pages we are releasing source code to these collaborators, but are not maintaining Symbian as an open source development project."

To use the Symbian source code, platform developers have to register with Nokia and use the Nokia Symbian License. The company said it will continue to offer development tools, documentation and other support to application developers.

The closing of the source code comes as Nokia begins to phase out Symbian, which has powered all its smartphones, in favor of Microsoft's Windows Phone 7. (Nokia feature phones run a separate OS, Nokia Series 40.) When it , Nokia said it expected to sell 150 million Symbian devices in the years to come. In the fourth quarter of 2010, it sold 28.3 million of them.