One of the little trumpeted but extremely valuable features in PowerPoint 2010 is the ability to compare and merge slides from separate decks. To use this feature, first open your main presentation file and go to the Review tab. Click the Compare button:
Select the file you wish to compare it with and click Merge:
Don't worry, the changes from the second document won't be retained unless you review and accept them.
Now look at the Revisions pane on the right for a list of overall changes:
And also any changes to the selected slide:
Right click changes you wish to keep and choose Accept Change:
Or leave changes alone and they'll be rejected and disappear when you save and close the document.