We've got one workstation running CS3 and Photoshop, Illustrator, and InDesign will not let us export or save as. When you choose the menu command you can see the dialog for a millisecond and then it's gone. And you can't do anything because the program thinks it's exporting or saving and waiting for input.
I've deleted the preferences file. And I've tried to repair the installation and that had no effect. My next step will have to be a complete removal and reinstall.
Sounds like the dialog has moved either to the back or off screen (are you set up for multiple monitors?) I'd try minimizing or closint all other open windows to see if you can make it visible, then move or resize the hidden dialog (and my memory is this works best in Phtoshop) to see if you can get the programs to display it properly again.