Well, I just got back from Hawaii (WONDERFUL!!!!!!! I'll need to post here and in the Trip Reports soon) and we didn't have any problems with cell phones. We were in Oahu, Hawaii, and Kauai and had good coverage (only spotty in the mountains, as others have noted).

I have Verizon, to which I am fiercely loyal because they have the best coverage (as I found out on 9/11 three years ago today in the DC area ) and I had no problems at all in any of the islands. I'm on a national no-roaming/free long-distance plan but was still careful not to use it too much since I think they charge you according to the time where you are, so 6 pm on a Friday in Hawaii would be just that -- peak -- and not the freebie "midnight on the East Coast/off-peak" deal.

My boyfriend has Cingular and his phone worked well, but I worry if he may have had some roaming charges. There was note about "extended network" on his phone and his clock NEVER changed drom East Coast time, whereas my Verizon one immediately changed to reflect the time zone in Hawaii. Hope this helps!

ETA: Not-so-off-topic, I've gotta say that we saw the movie "Cellular" last night and it was pretty good! Far-fetched but still exciting and even with a bit of humor in it.