The "potential" big deal is, whether you have a pt. on board or not, is just that one fraction of a second that it takes to be distracting, interfere, get in the way of, etc. Unless you are the owner of the ambulance you are in, there can be a lot of damage done to a very expensive piece of equip. And I'd say some insurance companies would have a thing or two to say about it. There's already a lot going on while operating any emergency vehicle. The last thing you need is one more.
Now, I'm not on any side, for or against, the use of cell phone technology. I think it's a great tool to be used for communication for dispatch, mercy call in's, etc. But let's use a little common sense too.