We just got to be patient.
Though I do get the feeling that Bethesda stopped caring about ps3, I do think Dawnguard will be released for ps3. But I have a feeling that will take a year or so.
But it is better to wait a long time for a good game than a short time for crap.
I'm given to understand Bethesda has announced on Twitter they will be launching the 1.7 patch in Europe on monday. Given the patch is more Dawnguard-prep than bug-fix, you've got to assume they intend to release the DLC... why would they put the effort into the patch if it were nothing but a smokescreen. If they were completely un-bothered with the PS3, they wouldn't bother. We have to assume the wheels are grinding slowly, but they are still grinding.