While I have no interest in defending Beth and their inept handling of all things PS3, it's only fair to point out the above. Plans change and the DLC was not built during the 'pre-release hype'.
But to say less than 48 hours before this announcement that it was still part of the plans? The decision to abandon future support of the platform isn't a snap one, this has been in discussion for weeks, possibly even before Dawnguard released on 360. It's fair to say that last August when Bethesda revealed their DLC plans and exclusivity that PS3 was definitely intended to be part of that after the exclusivity, but when Tweet Hines claimed on Twitter less than three days ago that Dawnguard was still coming to PS3 he was lying, there's no two ways about that one.
I don't expect Dawnguard to come out at all at this rate. This announcement looks to me like a ploy to piss off the entire PS3 community and let them vent their spleens at Bethesda now, so that when they finally grow a pair and officially announce that the PS3 DLCs are cancelled they can point back and claim to have posted updates on a decision that was made months before.