Apple will hold an invitation-only event at its Cupertino, Calif., campus this week at which it's expected to release the SDK.

One burning question: How much freedom will developers have? Some sources say the company plans to institute a strict approval process over any and all external iPhone applications. Others suggest only applications with price tags will require Apple's sanction and free applications will see a far less stringent approval process, if any. Such processes could make it difficult for vendors of iPhone apps to sell their wares, as Apple would then have a hand in what functionality they can offer. And it's likely that all third-party software will be sold through its iTunes store.

Regardless, the release of the iPhone SDK will represent a significant step toward the iPhone becoming a viable business device, says Forrester Analyst Chris Silva.

"This is Apple finally, though with some reservations, opening up its platform to vendors that make business-specific applications," Silva says.

Which applications, or enhancements to existing applications, are business users most eager to see? Here's our take on the top five iPhone application wishes:

1) Robust E-Mail Client With "Push" Mail Functionality

One of the biggest complaints from corporate iPhone users is that the device doesn't currently support "push" mail (mail that is delivered instantly as it lands in inboxes instead of being fetched by the mail client at set intervals). BlackBerry maker Research In Motion set the bar high with its tried-and-true push offering, and Apple will have to meet that challenge if it hopes to provide the level of service business users want and need, says Frank Mahdavi, Chief Strategy Officer of MIR3. MIR3 currently offers an intelligent notification application for the iPhone and the iPod touch, and Mahdavi himself is an iPhone user. Mahdavi says he expects that some of the first applications developed using the new SDK will be enhanced mail clients with push e-mail.