UPDATE: To see what will and won’t be in the new iPhone OS when it’s released this summer, you can read our summary here or follow our link to the Quicktime video here.

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The first thing to remember about Apple’s (AAPL) iPhone 3.0 special event, announced last Thursday and scheduled for Tuesday at 1 p.m. ET (10 a.m PT), is that it has been billed as “an advance preview.”

That means no new software — and certainly no new hardware — is likely to be released to the public today. What Apple is hosting an invitation-only event for developers so they can learn about a new iPhone SDK (software development kit) and get a sneak peek at the third major build of the iPhone’s basic operating system.

Steve Jobs hosted a similar event on March 6, 2008 — the “iPhone Software Roadmap” — at which he released a beta version of the first SDK and previewed iPhone 2.0. Both represented major advances over the original iPhone, but end users didn’t see the benefits until four months later, with the release of the iPhone 3G and the launch of the App Store on July 11, 2008. iPhone 2.0 introduced dozens of enhancements (and more than a few problems), but the most important advance turned out to be the SDK and the tens of thousands of third-party iPhone applications it has since spawned.

So what can we expect from Tuesday’s event?

Most of the advance speculation has centered on iPhone 3.0. Among the new features rumored or expected — but, remember, not necessarily available right away — are:

• Copy and paste. Better two years late than never, according to multiple sources. See, for example, here and here.
• Push notification. So Facebook, say, could alert you when you have a new message. This was promised in June 2008, but not yet delivered.
• MMS — Multimedia Messaging Service. So you can forward those pictures sent to you by friends with far less sophisticated cell phones. Maybe yes, maybe no.
• Better mail program. Why can’t you search past messages? Read them in landscape mode? Delete them en masse?
• Internet sharing. For those times when your iPhone has access but your laptop doesn’t. Apple and AT&T have both said so-called “tethering” is coming real soon now.
• Bluetooth support. Currently available only for phone headsets. Could be expanded to support wireless keyboards, speaker systems, file exchanges, syncing etc.
• Flash support. So you’d see videos and dancing advertisements instead of those little blue cubes. Adobe (ADBE) says its Flash Player software is ready and waiting for Apple’s approval.
• Better App management. The current interface is barely capable of organizing 148 applications, never mind 28,000.
• Voice dialing and turn-by-turn directions. Quick, before iPhone users cause any more traffic accidents.
• Video capture. It can be done without modifying the built-in camera as iPhone Video Recorder, an application available only for jailbroken iPhones, has shown.
• A new browser. The current version of iPhone Safari is nearly two years old and starting to get a little long in the tooth.

Owners of competing smartphones snicker when they read these lists; Microsoft’s (MSFT) Windows Mobile and Research in Motion’s (RIMM) BlackBerry have had many of these features for years. According to one report, Apple’s primary motivation for introducing the improvements now is to head off competition from the forthcoming Palm (PALM) Pre.

But what none of the competing smartphones have is an SDK that’s as easy to use as Apple’s or an App Store that makes marketing and distributing applications so painless — and profitable.

Which is why the first part of Tuesday’s event — the opportunity for developers to learn about the new SDK — may turn out to be the more significant.

As Seth Weintraub notes in his widely circulated Computerworld blog, developers are going to need some lead time if Apple plans to introduce a new piece of mobile hardware — an Apple tablet, as he suggests, or a more advanced iPhone or iPod touch.

Much less has been written about what that new SDK might include (and most of it is pretty technical), but among the improvements developers are looking for are:

• Better syncing between apps. So those 28,000 applications could share data among themselves.
• Better calendar and t0-do list support. So an e-mail invitation could be automatically added to your iCal.
• More background operation. So you could check your e-mail, for example, without interrupting that Internet radio show you were listening to.

After that, the requests quickly go over this reporter’s head. Ars Technica contributor Erica Sadun, for example, is asking for improved AVFoundation, CFNetwork frameworks, expanded UIKit objects and an improved Interface Builder (link), but we’re at a loss to explain what all that means.

In any event, our questions should be answered soon enough. My colleague Jon Fortt will be live-blogging the show for Fortune.com, as will several other sites. (9to5 Mac says it will be displaying four live-blog feeds at once.)

We’ll be monitoring the special event from our leafy backyard in Brooklyn and will post a summary of the key findings as soon as we know what they are.

See also: What’s on your iPhone 3.0 wish list?