So I went to the .Mac System Preferences panel, clicked the iDisk tab, and clicked on the Stop button in the iDisk Syncing section. Once you have done this, it should show Start to indicate that syncing is now off.
Once I'd done that, the computer appeared to be generally more responsive ... and when saving files, it takes only about a second to load up the dialog box. Much nicer!
[robg adds: A friend confirms that when he used auto iDisk sync, he also experienced slow Open/Save dialogs. I'm not sure if leaving it enabled but setting it to manual would avoid the issue or not...]

