I found out some etiquette things when I hosted my sister's baby shower. Like, apparently, because I am family, I wasnt supposed to host it (even though I am her 'best friend'). It wasnt until I was AT the shower that my aunt told me about the tradition of the father, the inlaws and the grandparents (perhaps this is a regional/cultural thing?)
My post assumed that the cards were addressed to you and your husband (I have only seen it done this way, actually) and so the gift was for both of you. I would assume that even if it was addressed only to you, this was a mistake on the part of the giver, and etiquette would still require both of you to thank the giver, but JudiAU might have a point. My sister also plans to sign her cards "from XX, XY, and Baby [lastname]", I think it is cute, but, I speak from my limited experience, I dont know what actual etiquette requires - perhaps the emily post website or a book from your local library/bookstore would help?