I used to work at an electronics store in Australia. I've transported many, many plasmas and LCDs sitting on their back, and some even on their front. I've set them up and turned them on straight after unloading, with no ill effects.
In the early days, plasmas had to be transported upright. These days they don't. I've even seen in a Panasonic manual, an instruction to rest the TV on its screen (on a cushion) in order to put the stand on. If that's not a green-light for putting your plasma horizontal, then I don't know what is!
I know American-market TVs aren't as advanced as Australian-market TVs, but I don't think the Americans are three years behind :-P