Don't buy it.
The US already had bases in Saudi Arabia and Turkey, and could easily have sent an aircraft carrier or two to the gulf at any time if Iran started sabre rattling. There was no military justification for the invasion. Indeed Saddam was helping to keep Iran in check. That's primarily why the US supported him in the eighties.
An air craft carrier is not enough to take on an entire military. We would need a REAL presence there, one that we could never have had in Saudi Arabia or Turkey. Saudi Arabia is clearly playing both ends against the middle and Turkey isn't much of a military staging ground when they keep revoking permission to use their airspace.
The type of installations now built in Iraq completely dwarf anything we had in the Middle East 5 years ago.
Forgot to mention Afghanistan. Oh, and Pakistan's Musharraf has shown he's open to bribes.
But that's beside the point. A full scale invasion of Iran is not necessary at this point and would almost certainly be counterproductive in the longer term.
If Iran is suspected of developing Nuclear Weapons and refuses to cooperate with UN inspectors then the most proportionate response would be to destroy the sites where the weapons are being developed, either from the air, or by commando raids. If the sites cannot be found (which I think is unlikely) then punitive raids on military targets could be carried out until Iran let inspectors in. This was broadly the approach adopted towards Iraq for 12 years and I believe that by and large it worked.
However, if Bush was foolish enough to launch a full scale invasion of Iran then finding ways to get his troops to the border would be the least of his problems. For a start where would he get the extra men from? He already hasn't got enough to secure Iraq and Afghanistan. Not even Blair would help him this time. He'd probably have to consider reintroducing the draft and that would certainly concentrate the minds of US voters. After the invasion there would be an insurgency of such ferocity it would make Iraq seem like a picnic in comparison.
And what would be the exit strategy? There's no point in trying to impose democracy on Iran as it's already (sort of) democratic. OK it's not really democratic by western standards but the wacko currently in charge was actually voted into office.
Incidentally, when it comes to dealing with Iran's alleged nuclear weapons programme the west is once again guilty of double standards. Israel developed its own nuclear weapons many years ago and yet no action has ever been taken. I'm sure the Arab world would find western intervention in Iran far more acceptable if the issue of Israel's nuclear weapons was dealt with at the same time.