I'm definitely in the "let them know from the get-go/first date" camp.
I generally meet people via OKCupid. It is clearly stated in my profile that I'm poly and it is linked to my husband's profile. His is the same way.
Now, I also think that not mentioning poly or any primary partners until the first date would be fine in my book, but like a few others I would feel deceived/uncomfortable if I went out on a date, or even just a get-to-know-you meet for coffee, and a person in a committed relationship didn't tell me about said committed relationship(s).