In an email sent to developers yesterday, Apple has warned that all new iPhone applications submitted to the App Store must now be compatible with the 3.0 OS as well as the current 2.2.1 OS that is in widest circulation. The announcement came as it released the fifth beta for the upcoming firmware update, which was revealed during a press event back in March.

Apple also disclosed that any existing application not running properly under 3.0 may also face deletion from its App Store once the update launches. Unsurprisingly, many developers have expressed concern over this move. Not only are they being forced to tweak their existing code without knowing an official release date for 3.0 (probably around Apple’s WWDC event in June), but also must spend time testing their apps against an unfinished OS base.

The company does note in its email, however, that most titles in the App Store are likely to run on the new operating system without modification – although there are some developers that beg to differ.