Well, my $.02 is that this will encompass most of Asia Minor and Asia proper. The IDF has already said they'll be employing ground forces across the border. That was part of the reason behind the leaflet drop the other day, to get civilians out of there. Syria's already said they'll come to the aide of Hezbollah and Lebanon against Israel, plus they're still housing the leader of Hammas, who's holding a soldier hostage.
Israel has already performed a supersonic, low-altitude fly-by of the Syrian president's summer home. Once it becomes inevitable that Israel will hit Syria, they'll likely use the WMD's that I believe Russia moved from Iraq to the Bekaa Valley. Once that happens, watch the Jericho missiles fly. We'll get involved since Iranian ground forces have to go through Iraq to get to Israel.
North Korea could get involved since they have a deep military relationship with Tehran, and someone pointed out the other day that from Pyongyang to Sacramento is 5600mi, but from Pyongyang to Israel is a hair over 4900mi.
Whether or not that happens, I think both China and Russia could get involved, too, because of their respective dealings with Iran, and their desires for regional influence.
i'm still not convinced Olmert has what it takes to actually invade southern lebanon. If he does, Syria will definetly feel the pressure to intervene, if they don't intervene at that point they lose serious face, if they do intervene they will get their rear ends handed to them and we will have to see if they launch any of their missile arsenal (which is likely a lot more potent than what Hizbullah has) at Israel. At that point all bets are off