Thank you so much for all your kind and considered replies.

I don't think it is fair to punish the child who is having the party by not allowing the invited guests to go. More often than not, the kids don't get a say. If they want a party, the parents say it is here, or none at all.
I think this addresses what (I now know) I was getting at. I feel really bad for the kid having the birthday. What to do?

I wouldn't be offended, but I wouldn't mind knowing why, either... if you just said no, or that you don't 'do' them it might leave me wondering what the problem is (asusming that if theoretical 'I' am holding the party, I don't have a problem with it)
I wouldn't think you have to go into great detail, just a basic ... '_' .... erm, I don't know what.. just so they know it's not personal?
I guess I also wanted to “put it out there” that there are parents that have really strongly held views about this. Sure it’s not like it’s a religion or anything but this is fundamental to how we as a family live our lives. So, I should explain a bit more?

I guess I’ll only be able to veto for a few more years anyway. Soon DS will be old enough to know what’s going on and then I’ll let him decide for himself if he wants to go.

I can't say whether I would be offended or not if a parent declined an invite should I ever throw a McD's or HJ's party. To be honest, I probably *would* be offended for a minute, and assume that the parent in question is just one of those snotty so-and-so's who thinks they're better than all the other parents for being 'above' junk food, junk toys and 'lazy' parenting
Bwahahahaha.... you’ve summed me up to a tee! (no seriously I’m not quite that self-righteous, especially when it comes to using the TV as a babysitter ).

And just on the, “well we went to them and it never did us any harm”. Hmmm..... I dunno about that one either. It might not have done you personally as an individual any harm but just look at the obesity epidemic in this country, and not to mention complete lack of awareness about what is in food, where it comes from, how it’s produced and all the vested interests there are in getting it to our mouths. I think this sort of thing is all part of the immeasurable damage being done to us as a society... and I rage against it.

Anyway, thanks so much again for letting me chew ( ) this over with you all. I very much appreciate it.