Apple is no longer accepting any iPhone or iPod touch applications that are not compatible with iPhone OS 3.0, developers have learned. Some of the existing apps are reportedly susceptible to being removed, while those that are not compatible with OS 3.0 will need adjusting to continue to be supported by Apple.

In short, every iPhone developer needs to begin to develop for iPhone OS 3.0 starting now. TUAW reports that, according to an email sent to developers, Apple has informed them that, “Beginning today, all submissions to the App Store will be reviewed on the latest beta of iPhone OS 3.0. If your app submission is not compatible with iPhone OS 3.0, it will not be approved.”

The site makes a note of its own, saying that OS 3.0 does not yet have a release date, while “developers might be pushed to spend time and energy tweaking apps against an unstable OS base.”

Free application developers are the most affected, as Apple's OS 3.0 is accessible only after paying up $99 to join the developer program. Yes, we may see less free apps hitting the App Store starting now. Nevertheless, Apple does reveal that the existing ones should already be compatible with OS 3.0, additional testing being more of a security measure.

“Existing apps in the App Store should already run on iPhone OS 3.0 without modification, but you should test your existing apps with iPhone OS 3.0 to ensure there are no compatibility issues. After iPhone OS 3.0 becomes available to customers, any app that is incompatible with iPhone OS 3.0 may be removed from the App Store,” the e-mail sent out to developers yesterday reads. Apple then goes to offer download links to the latest iPhone OS 3.0 (and SDK) Beta, encouraging devs to start testing today.

It will be interesting to see how many developers will refuse to pay the enrollment fee and wait to see whether or not their app is still supported in summer. However, sooner or later, everyone will have to hop aboard the OS 3.0 train.