Are there informed speculations about what is the next Mac that is going to be upgraded? There are five lines of Macs (MacBook Air, MacBook Pro, Mac mini, iMac, Mac Pro), and AFAIK Apple has almost always updated only one at a time, leaving the others a bit behind.

Now, it should be easy to keep track of the most recently updated, hence the odds of an update for the older models increases, and this is what I would call an "informed speculation" at a very basic level.

Is there any site which aims to keep tracks of these updates, and possibly adds commentary on product line changes?
MacRumors Buyer's Guide seems to provide the best predictions for updates. They use the average timescale for updates to provide a guess as to whether an update will come soon. They also include prominent rumours towards updates.