A strong argument can be made that Ankara feared that Washington might stab it in the back -- literally -- by facilitating the establishment of an independent Kurdistan on its southeastern border. The Kurdish issue goes to the heart of Turkey's internal and external security, so I really don't blame them for this decision.
Turkey may well have feared this potential. Everything I have seen claimed that in return for the land passage the US was offering:
1. $ Aid, and
2. A promise not to establish an independent Kurdistan, and not to leave heavy weapons with the Kurds.
There was much gnashing of teeth that we were "abandoning" the Kurds.
Now it looks like the Turks will probably stage their own invasion of Iraq while the US comes in from the south.
What happens afterward when the US politely asks Turkey to leave?
Seems to me that the Kurds will willingly provide the manpower to help the Turks back to their own country if the US just provides them the means to push things along. (If they can ever quit fighting among themselves over who is going to be in control.)
Suddenly Turkey has managed to generate its own worst nightmare, by refusing passage to the US.
The Kurds are there. That is reality. When Saddam's government falls, what does it matter whether the Kurds obtain soveriegn rights or are governed by a distant, disconnected government in Baghdad? Turkey needs to get over 1925 already. As an example, the European nations that fought against each other are getting along well enough.
PS: I do in fact respect Turkey's decision, and the consequences that accompany it. In my opinion, France's treachery in undermining our self defense interests is NOT forgivable.