I feel like you're the only one who "gets it." That said, I was about to post something similar!
I especially like how you picked up on Apple not supporting the older hardware in iPhone OS 4.0. I've been saying that for awhile and believe it is very likely, especially when they add in support for multitasking, which I will get to later.
This is how Apple announcements go, and they've left room at the bottom of their line in the iPhone and iPod Touch space for these bumps. This also makes sense as it was announced recently that Toshiba had found a way to double their capacity for cheap. EOL'ing the iPod Classic makes sense also, as a 128gb Touch would replace it quite well. iPod Touch with camera is an almost certainty at this point.
As for iPhone OS 4.0, I'm not sold on a release in January, but it' is possible (more on that in a minute). Apple usually announces the next version around March, makes the SDK available in beta for several months, and then releases the final version along with the iPhone updates in the summer. I would think that Apple could, perhaps, preview the coming OS update. Although it has been awhile since 3.1.2 was released--three months in a week.
If there is no iPhone OS update by the end of January then we might expect to hear something. Looking back on the timeline of iPhone OS updates, the last update before 2.0 was February 27, 2008 and then it was updated to 2.0 on July 11. The last update before 3.0 was on January 27, and it was updated to 3.0 on June 17th. I know it's only 2 years worth of data, but they've stayed around 4.5 months between the last update and the next version of the OS. If we don't see an update in January, then it is likely that the October 8th update to OS 3.1.2 will be the final update, barring a security patch on the scale of the SMS vulnerability.
So what does this mean? Well it might mean nothing, or it might mean that that iPhone OS 4.0 is coming out around Valentine's day. Given that the event is on January 26, Apple might do a delayed release making it available two weeks later. What I'm saying is probably completely wrong, but it is plausible, especially if Apple has gotten to the point where they can release 4.0 and it won't mess with developer's existing code, therefore not requiring an advance notice. This also plays to their advantage, 1. They won't have 125,000-ish apps resubmitted for approval based on 4.0 standards and 2. The older iPhone hardware wouldn't play nice if everyone made their apps 4.0 required (because of the lack of support for the older hardware, which I'll also get to briefly).
In addition, it is also possible that the tablet device could be running 4.0, and that this version is only supported on iPhone 3GS / 3rd gen iPod Touches as well due to higher hardware requirements. Could we see multitasking? Perhaps they have gotten their power management under control during the tablet development. However, on the iPhone and Touch it would probably see a limited implementation--such as 3-5 3rd party apps running at a time. I would prefer to see this done in the style of how mobile Safari handles tabs, similar to the Palm Pre's cards. The other day there was a JailBreak app released that does this beautifully.
Finally, the Macbooks and Pros. I doubt they would go with a silent update if they update the processor, as these will be updated to the Intel Core i3 and Core i5 processors, which is kind of a big deal. nVidia is still fighting with Intel over chipsets and even the feds have gotten involved. The integrated graphics of the new Intel chipsets is below the performance of the current nVidia 9400 series, so if we see updates in the processors, we will also see discrete graphic cards in the Macbooks and 13" MBP. This would also warrant further need for something more than a silent update, and Apple would probably play it up to be some big advancement to finally have discrete graphics in their lower end offerings and how they did it for performance and the consumer. I give this two possible scenarios, as I'm sure Apple has already made decisions to go ahead with these new processors. Either 1. They announce new updates in January and make them available by the end of Feb / beginning or March, or 2. They simply wait and hold a press event around the middle to end of March. Maybe nVidia will be allowed by then? This second scenario seems more likely, as the Mac Pro recently had a silent update and theoretically more time should pass before it is updated. In addition, it would probably be a more significant update to the new Intel 6-core processors. This would also be a good time for minor speed bumps in the iMac line and perhaps even the Mini.
So there are my long-winded thoughts and predictions. I'm just using simple logic based on what Apple has done before, patterns we've seen, industry news, and a sprinkle of rumor. Nothing fancy or groundbreaking. All the information is out there. I'd say that I'll probably be at least 70% right when everything is said and done. Sometimes things get pushed back to later events too. I mean there has to be some crap to show at WWDC, besides a 10.7 preview. But it will be fun times until then!
