interesting discussion
i have boy/girl twins who have grown up obviously exposed to the same experiences and parenting- the ultimate social experiment!!!!

I firmly believe that their interests are inate from my experiences- for instance they both have the same toys to play with. Their toys were never mine or yours- it was always they are ours! Everything from dolls to cars and their levitate to those toys they feel most comfortable with- cars for my son and dolls for my daughter.

They do not have cousins that they see often enough to know what toys are boy//girl toys, nor were they put in child care or hung around with older kids that they mimic.

Their interests have formed through their own decisions and choices and no matter how many different toys they have. My son loves constructions and my daughter likes dressing up. They do play heaps together and make their own games up, but their interests whether i like it or not, are 'typically' gender based.
Now that they are at school, they obviously are influenced by other children, but still they make up their own minds what they like and dont like- and i guess that is more important that what they actually play with.
just my food for thought
odette