UPDATE: As of Deja Vu 4.3 for Mountain Lion, items may be excluded in the Options > Exclusions tab. The information below is provided for the benefit of those who are using older versions of Deja Vu.
Although there is no user interface for this at the moment, you can still exclude certain files or folders from being backed up.
Use your favorite text editor (e.g., TextEdit) to create a plain text file named "excludes.txt", and add the full paths to the files and/or folders you wish to exclude from the backup (one per line). This text file needs to be saved with Unix (LF) line endings. Save this file in the "/Library/Application Support/Deja Vu" folder. For example, if I want to exclude my iTunes folder and my Trash, my "excludes.txt" file would look like this:
NOTE: At this point, exclusion paths containing "special" (non-ASCII) characters will probably not work.
UPDATE: Deja Vu 4 (Lion)
This has changed slightly in Déjà Vu 4. The new file is located here:
/Library/Application Support/Deja Vu/filters.txt
(It should already exist.)
Add your exclusions to that file, but begin each line with "- " or "exclude ". Like this:
You can read more details on this page:
(Scroll way down and look for the section called "FILTER RULES".)