From what i know, plastics manufacturing firms in asia are currently already producing certain components for one of the machines. (keeping that gem as vauge as possible, i don't want to be the source of a new batch of rumors across the net).
But i'll say here my own personal predictions based on my experience with the SDK's for both.
Pre-E3
Sony do a partial reveal of the platform with a sneak peek at software, putting the pressure on Microsoft
E3 2013 :
Microsoft reveal X720
Sony Officially Reveal PS4 @ Secondary project that involves the Vita and PS4 (SHHHHHH, sorry, did you think the Vita was already dead? oh well, no.)
Either neither company will reveal pricing or, if Microsoft's show is first, they'll announce speculative pricing and Sony will not
TGS 2013:
Sony announce pricing (if they chose not to at E3)
November 2013
Either just the Xbox 720 or both machines will be on store shelves.
December 2013
If it doesn't release at the same time as the 720, the PS4 will release now.
