In 10.6.6 I note the following:
Though one can add pdfs to the sidebar this merely makes a pdf browser out of Preview and they still cant be 'saved as' - or published via print pdf - as a MULTIPAGE file - UNLESS when dragging a pdf into the sidebar - you hover over the icon of the page thumb that you want to merge with - and THEN see a blue outline. Further drops are similar and you can drop within the blue outline so as to be in the order you require. When the pages are grouped thus the thumb shows an arrow - which if clicked displays the other pages.
hope this helps - Brian

thanks, brian!

your hint pionted me in the right direction :) the updated directions for merging pages from multiple PDFs into a single PDF using Preview 5.5 which comes with in 10.7 are:

Use Preview to open up the two PDFs you would like to merge. Choose View » Thumbnails. Make sure both PDFs are visible on the screen at the same time. When the Thumbnails sidebar becomes visible, you will see a graphical representation of the pages in your PDF document. Simply drag the page or pages (hold Command while clicking to select multiple pages) from the Thumbnails of one PDF to the Thumbnails of another. You'll see a green plus icon when Preview is ready to add the content from one to the other. Choose File » Duplicate and then Save and you wil have now merged pages from two separate PDF documents.