OS X has been on sale since March 24 at $129, but hasn’t been included on new Apple hardware until now. It will be installed, along with the older Mac OS 9.1, on all new machines leaving the company’s factories. Customers buying new machines that don’t have OS X on their hard drives will receive a boxed copy of the software.

By default, though, the machines will still start up in Mac OS 9.1. Users who want to run the newer system, either occasionally or on a routine basis, will have to change a control-panel setting and restart.

Apple CEO Steve Jobs made the announcement during a “fireside chat” — with a fire burning brightly in a digital representation of a fireplace on a large projection screen behind him — opening the company’s annual Worldwide Developers Conference in San Jose.

Warning that “the train has left the station,” he used the occasion to urge developers to step up their efforts to deliver new or updated applications that take advantage of the capabilities of Mac OS X.

A few major Mac programs, such as Macromedia’s FreeHand illustration program and the FileMaker Pro database program from Apple’s own FileMaker subsidiary, have recently been revised for the new system, but most Mac applications, including such standards as Adobe Photoshop and Microsoft Office, have not yet been updated.

Mac OS X, based on the industrial-strength Unix system, can run those and most other existing Mac programs, but until they are updated, they run only in a “classic” mode that lacks the new operating system’s key features, including improved stability and a colorful new user interface.

The new system requires 128 MB of memory, but Apple said it will be loaded even on entry-level iMac and iBook configurations that ship with only 64 MB.

Apple also announced a new release of Mac OS X Server, a version of the new OS intended for machines that serve up Web pages, files, e-mail and streaming video. The company sells the server version mainly to schools and to customers in the creative-arts market.

A version of Mac OS X Server that supports as many as 10 Mac clients is $499, while an unlimited version is $999. Apple is also offering the software loaded on two hardware configurations, priced at $2,999 and $3,999.

In addition, Jobs said that Apple has become the first major computer-maker to eliminate traditional cathode-ray-tube technology from its monitor line. The company will now ship only flat-panel LCD displays, including a new 17- inch version due next month and priced at $999. It cut prices on its 15-inch flat-panel display by $200 to $599, and on its 22-inch model by $500 to $2,499.