Hi Bob,
I got my iPad mid way through last year. It's hooked up with Verizon's 3G coverage, and I took it with me on a few of my hikes last year to see exactly what you want to know.
It worked great on most peaks through the Mt. Baldy area, except for Mt. Baldy. The 3 T's, Cucamonga, Ontario, and Big Horn all got me connected.
Around Mt. Whitney, it was very spotty. Up to Lone Pine Lake I could get some 3G action, but not enough to stream videos or anything. I was able to send/receive emails to my wife though. Up to Trail Camp I got two messages to go through, but that was it. Mostly dead. Nothing above there, and I didn't summit with it.
Up north around the June Lake Loop, I got coverage a lot more reliably in a lot more places. All through the Ansel Adams Wilderness seemed to work great (at least at my destinations, I wasn't hiking with the iPad in my hands or anything). At Thousand Island Lake, we watched a 2 minute video about Thousand Island Lake, and we were able to email my folks about our hike while we were still on it.
Nothing that I want to make a habit of, but I did want to know what the limitations were. I doubt that I'll carry my iPad into the backcountry again, especially since I can't imagine that my iPhone won't get service in just as many areas.
