For what its worth, I don't draw any conclusions from the PHBII selling out its printing.
First, at least for the life of this edition, its an evergreen book. Of course it was going to sell out its first printing. It will probably sell out its second, sooner or later. Maybe even a third. PHBIIs will be being purchased three years from now, and no one wants to be storing them this far in advance. So new print runs will occur.
Second, I didn't buy the PHBII because the entire contents of the PHBII is available on the DDI, in the character builder and in the compendium. This has got to have an effect on sales, but WotC did still get money out of me for the DDI subscription, and the fact that books like the PHBII were going to be in the compendium was a big part of why I paid for it.