Mark your calendars for the annual developer conference in San Francisco, but expect more than just the iPhone SDK on the agenda.

Apple on Wednesday started notifying its ADC (Apple Developer Connection) members about its annual World Wide Developer Conference, also known as WWDC. The event is scheduled for June 9 through June 13 at the Moscone Center.

Marketed as, "Everything Mac. Everything iPhone," the annual pilgrimage is expected to draw a record number of developers as Apple is expanding its developer community with both the Macintosh operating system and the iPhone.

"With the development opportunities of the iPhone platform added to those of Mac OS X Leopard, this year's Apple Worldwide Developers Conference promises to be the most exciting, informative conference ever. Don't miss this chance to see the latest technology and meet the engineers behind the two most innovative platforms for your creativity," Apple said in its e-mail blast.

The iPhone has become the hot developer tool of the spring with the added support of Microsoft Exchange, a software developer kit (SDK), a distribution model in its new App Store, and a $100 million infusion of capital cash from KPCB. Reaction to the iPhone SDK has been strong. Apple reported more than 100,000 downloads of the SDK in four days since the announcement.

Remember, June also is when Apple said it plans to distribute version 2.0 of the iPhone OS, which will be available to users at no charge. While it's no guarantee that WWDC and iPhone version 2.0 release won't coincide, the odds are in not favor of the stars aligning on this one as Steve Jobs said the upgrade is expected in late June.

Introduced March 6, the SDK provides developers with the same set of application programming interfaces and tools that Apple engineers use to create native applications for the smartphone and iPod Touch. Both use the same mobile version of Mac OS X.

The upgrade is expected to include support for applications built with the SDK. Apple also plans to make available at the same time for a "nominal fee" a similar upgrade for the iPod Touch.

This is not to say that the Mac OS is fading off into the distance. The Leopard (Mac OS X 10.5) and inclusion of Intel microprocessor-based products helped Apple increase its desktop and laptop shipments 39.3% from a year ago.

Because it's Unix-based, Leopard offers a number of features that could appeal to business IT managers. Among other things, Leopard is compliant with POSIX (Portable Operating System Interface), a standard designed to ensure that code authored for one Posix system will work on another.

Leopard also provides a Terminal application that advanced users can employ to access the operating system's underlying Unix interface. The software also will natively run 64-bit or 32-bit applications.

Beyond its Unix credentials, Leopard comes with a preinstalled version of Boot Camp -- software that allows users to boot Windows on a Macintosh.