Just now seeing this post, I forget to check this board often.
We too had to see a social worker before we were cleared to do the IUI with donor sperm. Like Wookie's case, they wouldn't have stopped anything depending on our feelings about telling or not, it was just to flush out our emotions. DH & I had already discussed all this before we met up with the lady, but it was still helpful.
As far as telling, our immediate thoughts before we got pg, was to not keep it a secret. Our close family & friends know, but there is some hesitation from MIL, where she doesn't want us to tell anyone else ever, not even our son. Kinda odd in that she 100% supported us, and even helped financially to cover some of the costs when the insurance stopped coverage. But it is our decision, just as it is you & your DH's decision. And each of us has our own choices to make on telling & when to tell. I still feel that for us, we will tell our son. I honestly don't know when is the right time or how exactly to bring it up. Right now, we talk to our son daily about all sorts of stuff, and we've mentioned with hugs & cuddles how much we love him and that we went through so much to have our little blessing. He'll be 18 months next week, and I think that's about all the info I need to provide now that he'll understand. Next year, at 5 years, etc, I don't know.
Also, I forget most of the time that DS is not biologically my husbands. They are so crazy about each other, and such a joy to watch. When we chose the donor, we did choose someone with the same ethnicity and he had similar physical characteristics to my DH to.
Good luck!!! If I can help in any way, just PM me if I don't see these posts to answer questions.
Wookie - big congrats on your pg!!