Wow, I'm surprised that so many people feel that a cell phone is a necessary accessory for a pre-teen. Good to know. The reason I thought of asking them not to bring it is because I thought that would be more acceptable than asking the kids to leave them on the counter (in effect, confiscating them).
The reason I felt they were inappropriate is that I didn't like how they were using them; calling a girl who wasn't invited to the party (granted, she kept calling back, and seemed to not be upset about it, but still, I think it's inconsiderate), prank calling a boy down the street; tossing the phone back and forth across the creek (it hit the ground a couple of times, but never got wet or in the mud, so I guess it's okay), and simply talking on the phone when they could have been playing. I suppose the scene at the creek was the best visual metaphore for pre-teens; half the girls knee-deep in the creek, catching tadpoles and frogs, and a couple of the girls on their phones up on the bank, talking to a boy from school who they, of course, don't like, but still want to call and tell him every detail. At one point I asked them to just tell him to come over if they wanted to talk to him (he lives two blocks away, he comes over to play sometimes, he's a cool fellow) but that elicited a couple of squeals to the contrary.
I figured the authority I have to ask to not bring cells phones is the same authority I exert over all electronic media in my own home--video games, tv, computer, etc. I don't want them bringing their laptop or their x-box, either, and I would feel well within my rights to ask people not to send these over with their kids. With all but one of these girls at the party, these are kids I've know for 8 years or more (and they are all 11-12 years old).
Well, thanks for the opinions. It's good to hear from people for whom a cell phone is more important. I would have never guessed that parents feel that a cell phone is that important for their kids, and a requirement to be able to attend an event, even a sleep-over at a friend's house. Good to know. This party definitely reinforced to my husband and I that our kids aren't getting a cell phone for several more years unless they buy it and cover the costs themselves, though.