DaveMage
Slumbering in Tsar
But who will be told?
Sales numbers, so far as I know, are only given to WotC. So unless they announce them, or unless somebody at a given retail store lets something slip (which'd only mean something for that store) we're not really going to know just how well the second round of Core Rulebooks sell.
Distributors will know. Distributors talk.

(And while distributors don't talk to us, they do talk to those in the industry, and word gets around.)
Will we know all the numbers? No. But we will certainly have a basis for comparison amongst some distributors from PHB1 to PHB2.
Paizo sends out e-mails that include their sales rankings of "Top Products from Other Companies". When 4E first came out, 4E stuff from WotC was on that list. I haven't seen a single WotC product there in quite some time. (Though, in fairness, I may have missed it, but I do look for it.)
If PHB2 sales are a dud, the game might be in serious trouble. On the other hand, if they provide an energizing spark, then the game may be just fine.