Apple (AAPL) is not a company known for vaporware — products announced long before they are ready and sometimes even before they exist. When Steve Jobs unveils a product, it’s usually ready ship that day or in a matter of weeks.

The exception: Major updates to Apple operating systems, like OS X Leopard, which Apple previewed 16 months before it shipped. In theory, this is done to give developers time to get up to speed. But it can also serve — as vaporware traditionally has — to suppress sales of rival systems.

Is that the case with iOS 4.2, reportedly set for release on Friday and now rumored to be pushed back to late November?

We don’t claim to be able to read Steve Jobs’ mind, but he might well have been thinking about the wave of iPad competitors about to hit the market when, two and a half months ago, he previewed iOS 4.2. This is the software update that promises to finally unify the iOS devices: iPhone, iPad, iPod touch. In particular, it’s supposed to bring to the iPad key improvements that many iPhone users have enjoyed since June, chief among them multitasking and better app management through folders. [Wireless printing was also promised, but there’s some question about what form that will take. See here.]

On Tuesday, MacStories reported that iOS 4.2 was set for release at 10:00 a.m. PST (1 p.m. EST) Friday. Now it reports that the release date has been pushed back at least one week and probably two “due to last-minute WiFi connection issues.”

UPDATE: On Friday afternoon, Apple made iTunes 10.1 — a precursor to iOS 4.2 — available for download, and shipped a new Gold Master of the operating system to developers. According to a report in The Telegraph the operating system’s new ship date is Nov. 24.

[Follow Philip Elmer-DeWitt on Twitter @philiped]