I'm in the Develop module in Lightroom 3. The secondary monitor shows a grid with multiple photos selected. When I change a develop setting, e.g. exposure +1, the setting is only applied to one of the selected photos (the one shown on the primary monitor). Is there a way to make Lightroom 3 apply the change to all photos selected in the grid?
Yes: hold down command (ctrl on windows) and the 'Sync' button on the bottom of the development pane (i.e., the lower right) becomes 'Auto Sync', which will apply any changes to the whole selection.
The switch on the sync button toggles Auto Sync on and off more permanently.
Matt's answer put me on the right track. His solution works and made me discover another solution that is a bit more obvious. In Lightroom 3 the Sync button has a little toggle button attached to its left side. Clicking that toggles the button between Sync and Auto Sync. When the button says Auto Sync, any changes in develop settings are applied to all selected photos.
An alternate solution is to copy & paste settings.
Right click on the image with the edits and go to Develop Settings -> Copy Settings. Check the settings you what you want to copy via the dialogue box.
Next, select all the photos you want to modify (shift click), right click -> Develop Settings, and then Paste Settings.
Keyboard shortcut on the Mac is CMD+SHIFT+C to copy and CMD+SHIFT+V to paste.