It's my opinion that if a child throws tantrums at home but behaves appropriately outside the home, then he's just testing boundaries at home and you are responding appropriately. If he throws tantrums outside the home but not at home then it's more manipulative behavior and he knows how to work you, and get his own way-- not a good thing!
I like Alines comment about the grown-up conspiracy! I don't agree with the current philosophy that 'no one disciplines my kid but me'. There are too many kids with no respect for authority of any kind.
I agree with this whole post, OHG. Top to bottom.
I read somewhere that kids who behave at school, with the sitter, etc. but not with parents are just letting go where it's safe to let go. They save it all up for the people who love them because they know it's safe with their parents to let it out. It's a sign that they know they're loved. An annoying sign, certainly, but ultimately a good sign.
And I'm betting this will come as no surprise, but I'm one of those moms who has no problem telling kids at the playground to cut it out and/or straighten up. I don't think I've ever had a parent have a problem with it (they usually thank me), but if I did, I'd tell her to deal with her kids and then I wouldn't have to. Harumph.
So rock on, OHG!