A new report suggests that iPhone OS 4.0 could be close to release, months before it would normally be expected. Apple has heretofore only released new firmware with new phone models. However, CNET reports that a “prominent” app developer says he’s got the new SDK and has updated his app already. In the past, Apple has provided developer previews in March for new OS versions that are then released in June or July.

Rick Broida, who writes iPhone Atlas for CNET, noted today that he had gotten an email from a dev claiming that he's got a hold of iPhone OS 4.0:

iPhone OS 4.0 is imminent. At least, that's the only conclusion I can draw from an e-mail I just received from a prominent app developer.

The message includes the following snippet: “Just wanted to give you a heads up that we’ve submitted an updated app for the new iPhone OS 4.0 software.

Rumers have swirled ahead of the expected Apple tablet announcement later this month. An event – which is still unscheduled and unconfirmed – is supposedly going to happen at the Yerba Buena Center for the Arts, with the date variously being set as January 26 or January 27. The French site Mac4Ever said that the tablet announcement would be accompanied by an announcement of an iPhone 4.0 beta, which is purported to include a “simulator” mode that would allow apps to be run on larger screens. Some developers were said to have been given beta SDKs ahead of the announcement so that they could prepare demos to be shown at the event.

The website iPodNN had noted speculation on Wednesday that the iPhone OS update was being held back until the tablet was announced because the build contained too many references to the still-unannounced device.