Oh MAN!!!!!, luvmykids. That drives me nuts.
I can see some hesitation on laurib's suggestion, but an objection to thanking people???? No way.
Appreciation is so, so important for encouraging more involvement. I think not appreciating enough (by name, by email, by newspaper, by carrier pigeon, by party, by anything...) is one of the biggest mistakes PTO leaders can make.
On laurib's question, I think that having the "insider's" get a party is an invitation to a "clique" reputation. I know very well that a very few people do a ton of work. And those folks deserve every accolade and every honor and money and jewels and manicures, etc. But -- like in every type of organization -- the leaders need to be very careful of impressions.
On the other hand, I think a great, huge, loud (even expensive) party for *every* possible volunteer is a terrific idea. The key, though, is that every person who did even the smallest thing needs to be invited and celebrated.
This is a key concept in my parental involvement talks -- and won't get justice here -- but your goaol in appreciation is not "fairness". You're not trying to say that "Mary volunteered her tail off, so mary should get a dinner party, while Sue only baked a cupcake, so she should get a card or nothing."
You actually want to do the opposite, if anything. As leaders, you want those folks who are just dabbling with involvement to be "wowed" by how much you appreciate their efforts. You're not measuring/grading involvement, you're making the loud statement that all involvement is great and we'll thankfully take any help you can provide.
So my vote: "no" on exclusionary appreciation. A resounding "yes" on wide-ranging appreciation, even appreciation with a big budget.
Tim