They say don't count your chickens before they hatch. They also say don't count on any rumors about new Apple products until you hear it from Steve Jobs (or Phil Schiller).
On the other hand, you sometimes have to take uncertainties into account in making plans for the future, rather than waiting for them to be resolved into news.
The rumored Apple tablet device is one such case. If the rumors are true, it's introduction will lead to tremendous change in the personal computer market.
It will enable applications for which the iPhone or iPod touch would be perfectly suited, if they just had a little more screen area, or if they had a few times the memory or faster processors. This by itself is huge.
It will get a jump start by running existing iPhone/iPod touch apps unmodified. This may be speculation, but it makes too much sense not to be part of the plan. Expect the resolution to be a simple multiple of that of the iPhone, like 640 X 960, or maybe 800 X 1200, maintaining the 2 X 3 aspect ratio.
As far as I'm concerned, the prospect of extremely fast cellular wireless internet connectivity is icing on the cake, but there are doubtless potential applications for which this is essential. Moreover, if tethering to your desktop is allowed, it will prove a huge selling point.
In fact, that issue of interoperability with other equipment remains a big question mark. Would an Apple tablet device work as a touch interface input device for your Mac? Would it work as a remote controller and/or handheld viewer for your Apple TV? Chances are the answer to both is "yes, with the appropriate application running."
If you're a (potential) iPhone software developer, you'll be wanting to keep the possibility of a larger device that works very much the same way as does an iPhone in the back of your mind, maybe even starting to accumulate the alternative resources you'd want to include if there were a possibility of your app running on a device with 4..9 (2^2 to 3^2) times the screen resolution, or making sure your app is resolution independent by using vector-based graphics.
To get back to the main point. The significance of the release of this device, if it happens, is too great to ignore just because it might not happen. If it happens (and it's looking like a good bet that it will) it will be HUGE!
(Here's Jason O'Grady's take on what an Apple tablet would need to be successful.)