I think the 9th criteria from the DSM-IV explains it:
(9) transient, stress-related paranoid ideation or severe dissociative symptoms.
So it's just one of the criteria and you'll see that some BPD will have this symptom while some won't. While mine didn't have complete psychotic symptoms, she did display lots of dissociative ones.
If the BPD is not diagnosed then it's not technically comorbid. That said people can present with a whole array of symptoms and behaviours that may be attributable to BPD, but for many reasons never receive a diagnosis. And of course they might not actually meet the clinical criteria for a diagnosis of BPD either.
What we know though from bpdfamily is that diagnosis or not, the impact of the behaviours and symptoms that bring us to these boards can and do have a devastating impact on all involved.
Absolutely! The diagnosis is irrelevant when it comes to being incompatible partner material for us.
Also SH, your avatar is Howl!