While trolling the Net for flat whistle info, I found a page on getting the weight of the ACR whistle (with lanyard) down from it's already svelte 7 grams down to 3 grams! (and getting the bulk of this already tiny whistle down, too!)
http://www.backpackinglight.com/cgi-bin/...thread_id=33706 Now, he has to whistle one-handed because he can't hold the whistle with his teeth. For me, saving a couple grams is not worth losing that feature.
A manufacturer seems to make a survival whistle as small as possible for its features. Side note, the Fox 40 Classic has some useless extra plastic because it's designed to look like a traditional referee whistle.