I don't think you should do it. You maybe thinking 'what they don't know can't harm them' but point is, you know and you know that if they knew they wouldn't be happy. Since it's their home, I don't think you should be using their utensils to cook pig-based products. It's dishonourable.
Also, part of morality is honour and trust between people and a certain level of respect for their beliefs (even if you do not agree with it).
Edit: just wanted to add the above point.
My thoughts too. It's just that I didn't act on them.
I wouldnt serve any muslim guest of mine anything not halal.
I agree with fererro... trust is very important. I wouldnt
put any meat or meat biproduct (i.e. bacon grease, or any kind
of meat stock for stews/soups)
in a vegitarian's meal, either, for that matter.
If it offends them, regardless of the reasons, why would you
want to do that?
Of course, me too. I would never put anything non-Halal in my parents food. I'm just talking about using utensils. Like I said, my mother doesn't mind me cooking non-Halal beef or chicken with our utensils. But I haven't asked her about pork (and would never ask her).