But covertly, that's another story. The Iranians are allied with the Syrians in Lebanon and now Iraq (which is interesting, since they were originally nominal rivals in that 80s conflict in Lebanon). I think, especially if they could trap us into occupying Saudi land, that we'd see a massive influx of funds and arms to whatever insurgents. That's a pretty safe guess - it's what they've been doing in Iraq for the better part of a year.
The asymmetric resistance we would see in Saudi Arabia if our troops had to occupy it, would be much larger than what we saw in Iraq. And more locals would meld into the resistance in Saudi Arabia than what occurred in Iraq.
That makes sense if the Arab rulers of Iran want to reflect back on Arabia to restore Islamic purity to the owners of Mecca. The thing I see is Israel being the major impediment to the Iranian plan, and the US invasion of Iraq keeping Israel out of the picture so as to preserve the oil fields from being nuked as Israel is overrun.
Good point.