Apple frontman Steve Jobs opened the iPhone up to developers on Thursday evening, with details of the much anticipated Software Development Kit (SDK) at an event held at the company’s headquarters in California.

As of yesterday, developers could download the beta iPhone SDK for free and run an iPhone Simulator on their Mac. The full version of the kit will not be available until June, when Apple releases version 2.0 of its iPhone software, possibly coinciding with the Mac World Wide Developers Conference.

Until then, developers wanting to write native applications for the device will have to make do with the emulator, or those living in the US can apply to become part of the iPhone Developer Program. Successful applicants will be offered the ability to get code onto iPhones for testing, at a cost of $99 per year for the Standard Program and $299 per year for the enterprise programme, which will be targeted at those building proprietary in house applications.

Developer Program members will be equipped with an iPhone SDK and development tools, access to pre-release iPhone software, technical support, the ability to get code onto iPhones for testing, and a distribution channel.

Raven Zachary, one of the founders of the iPhoneDevCamp, a sizeable iPhone developer community, was writing live form the event yesterday and said he thought the offer was, “good enough to attract thousands of developers.”

As promised, Apple also gave the iPhone more credibility for the business market, as Jobs previewed version 2.0. The next instalment will feature support for Microsoft Exchange ActiveSync to provide secure, over the air push email, contacts and calendars as well as a remote wipe feature, and the addition of Cisco IPsec VPN for encrypted access to private corporate networks.

Enterprise IT administrators will also get a configuration utility to easily and quickly discover and remotely set up multiple iPhones, including password policies, VPN settings, security certificates and email server settings.

A beta release of the version 2.0 software has already been made available to selected developers and enterprise customers, so it will probably be all over torrent sites by now.

Cunningly however, Apple is maintaining control of the distribution channel for applications. Version 2.0 of the software will feature the App Store, an application that lets users browse, search, purchase and wirelessly download third party applications directly onto their iPhone or iPod touch. “It’s Apple as the conduit,” said Zachary.

While this means developers can potentially get their apps in front of every iPhone owner, it also guarantees Apple a nice revenue stream. Developers can set the price for their applications but Apple gets 30 per cent of all sales revenues. It’s worth noting however that free applications are distributed without charge. And enterprise customers will be able to create a secure, private page on the App Store accessible only by their employees.

Not everything will be allowed however. Jobs said that apps which are “bandwidth hogs” would be banned, including VoIP apps which use the cellular network, although those based on wifi would be allowed.

To show support for the initiative, Apple unveiled an $100m investment fund from Kleiner Perkins Caufield & Byers that will be available to develop new iPhone apps. KPCB’s iFund claims to be agnostic to size and stage of investment and will invest in companies building applications, services and components. Focus areas include location based services, social networking, m-commerce, communication, and entertainment.

“A revolutionary new platform is a rare and prized opportunity for entrepreneurs, and that’s exactly what Apple has created with iPhone and iPod touch,” said John Doerr, partner at KPCB. “We think several significant new companies will emerge as this new platform evolves, and the iFund will empower them to realise their full potential.”

To round off the event, Jobs also announced some welcome updates to the iPhone that will come with version 2.0. These include mail features such as the ability to view PowerPoint attachments, in addition to Word and Excel, as well as the ability to mass delete and move email messages. Apple also showed off an AIM Instant Messaging client from AOL, although there was no sign of an iChat client from Apple. Of course, there was no word on Adobe Flash either.