Most of the items in your sidebar will appear in the open dialog in 10.5. One exception is the items under Search For, including your custom Saved Searches.
However, if you navigate to the ~/Library/Saved Searches folder (or any place you have Saved Searches stored) and drag the Saved Searches to the Places section of your sidebar, you can open (and even edit) saved searches from within the Open Dialog. Note that searches can appear in both the Search For section and the Places section at the same time; I dragged mine out of the Search For section, since they're more useful in the Places section.
