Just to add my two cents to this of what I think Wizards actually
will (and won't) include:
First of all, I don't think this is quite what everyone is thinking. It's not going to be the "one book you can bring to the table and use" - the description says it's a
compendium of all rule supplements and errata. Think
Spell Compendium and
Magic Item Compendium.
I surely hope they don't include monsters, spells, and equipment in this. The Spell Compendium and PHB covers spells pretty well. Every monster manual is (in essence) a "Monster Compendium" - and equipment is covered fairly well already, without being
too scattered around, especially with the new
Magic Item Compendium.
I think this will probably be a compilation of all rules and rule supplements they've ever included in all of their books. For example: Skills - there are skill uses in
tons of books aside from just the PHB, but no one knows them (or rarely uses them) because who wants to go look all of that up? Well, now here will be a quick and easy reference to all of the uses of every skill.
Combat will probably be similar, as well as any other
rules. Those are all simply core game rules. I even kind of doubt classes will be in there, though I would see them before monsters and spells and items. But I doubt it.
This won't be a compact rule guide, though. It will be a rule
compendium. All current rules in one place.
I know, I know... some of the discussion is on what people would like to
see in this...

But this is what I'm figuring it will actually be.