And if its a quiet update, its not going to be anything major... and if its nothing major, theres going to be a lot of disappointed posters on this forum.
No one has seen the pattern here?
Mac Pro and anything related; minimum press out it. iMac, Mac Mini, MacBook, MacBook Pro; medium to large press, you'll see a nice big splash image on apple.com, unless it's a major, major upgrade. iPhone, iPod; huge release.
Anyone care to guess as to why this is how it is? Think of it through a marketing perspective. Someone who wants a Mac Pro, as myself, knows what they want and will not settle for anything else. Huge customer loyalty. No need to persuade me, I'm buying it. The last release had the new processors, major architecture update and what did you see? Little button on apple.com.
iMac, Mac Mini, entry level Mac computers so they have the potential to bring in new Mac customers, unlike the Mac Pro, so there is more press related to this. But they can also be for the die hard Mac enthusiast so then it's a job of customer retention. Events won't be a common practice to release updates unless they're huge.
MacBook and MacBook Pro are kind of clumped into that last group but the technology behind a laptop is of more interest so you may see more press related to this. Lighter, smaller, faster, cooks your breakfast, etc.
iPods and iPhone is where Apple makes a killing. Everyone has one or wants one, so any chance to tighten that stranglehold, they'll take it. Hence the event to announce a new iTunes and a few features and new iPods. It's their gateway drug, so to speak.
With that being said, when they release an updated iMac, unless it's a phenomenal update, an event isn't really necessary.
Anyway, this is my observation and understanding. I could be wrong and they could release an iMac with four quad-cores and dual screens, blu-ray and a kitchen sink, but I doubt it.