@matthew: I've been summoned! (Neat that the system sends a message whenever you're mentioned in a comment.)
And, yes, I have the answer. NO, the Giant Bombcast has never failed to produce an episode within a Sunday-Saturday calendar week.
As far as I can tell, there has only been one Tuesday since the Giant Bombcast began on March 11, 2008, that there was no Bombcast episode, and that was October 14, 2008. However, there was an episode that week, two days earlier on Sunday, October 12th, the last Bombcast from Toyko on their trip for TGS 2008.
(Also, I should really look to see if I have messages more often.)