Dear Jay,
Anecdotally I have two patients with postnatal depression who have taken Zoloft and continued to breastfeed. One was on 100mg per day and the other 150mg per day.
Both these women had been tried on other antidepressants without success. The later I considered to be a significant suicide risk. Both were seen by psychiatrists as well as myself.
There is, as you have stated minimal information available regarding this drug and its safety in breastfeeding. I would obviously choose a tricyclic antidepressant as first line in any depressed woman requiring medication who was breastfeeding. TCAs have been on the market for four decades and  appear to be compatable with breastfeeding. However if Zoloft is chosen I tell my patients the following:
Zoloft is occasionally used in breastfeeding women and to date there is no evidence that it has posed a problem for their infants.
Unfortunately, however it will be 20-30 years before we can reasonably state that it is  safe to use when breastfeeding.
I can state however that there a huge number of proven benefits to both mum and her baby if she does continue to breastfeed. I might discuss them at length if it seems appropriate at the time.
It is therefore a matter of weighing unproven risks of taking the Zoloft and continuing to breastfeed with the proven benefits of continuing to breastfeed.
I tell them that it is their decision but state that if I were in this situation I would continue to breastfeed and take the Zoloft. (And I would!)
If she chose to continue to feed I would monitor the baby carefully for the sort of potential problems that Tom Hale describes such as vomiting, diarhoea, tremor and sweating.
I think that it is a tragedy when a women with postnatal depression is asked to wean. It is the one special thing that only they can do for their baby. Asking them to stop breastfeeding just confirms for them the suspicion that they are not a good mother, a viewpoint that they may never recover from.

Laureen Lawlor-Smith
Family Medical Practitioner IBCLC
South Australia