There is also the speculation that WotC is waiting to see how Essentials does. If it does really well they might revise the future of D&D altogether; if it does reasonably well, it will be business as usual; if it does poorly, we might see 5E sooner than we think!
I doubt it. The way it currently looks is that post-Essentials 'standard' products will be influenced by the design choices they made for the Essentials products. I.e. books will have feats descriptions similar to Essentials, class write-ups will borrow elements from Essentials classes, etc.There is also the speculation that WotC is waiting to see how Essentials does. If it does really well they might revise the future of D&D altogether; if it does reasonably well, it will be business as usual; if it does poorly, we might see 5E sooner than we think!
5e is something we're going to see earlier if Essentials products _and_ standard 4e products sell poorly.
On the other hand, I wouldn't be surprised to see this gap leading to a bit push later in the year. No, it wouldn't be "4.5" but it would probably be revised core rulebooks, which I would welcome at this point.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.