Setting Preview Default to Single Page
In Mountain Lion, at least, there's a Opening for the first time entry in the PDF pane in Preferences, where you can select Single Page. This sets the default view mode for all PDF files.

If you for some reason can't get it to work, you could try this Terminal command (don't know if it will work in Lion) and restart Preview:

defaults write -app preview kPVPDFDefaultPageViewModeOption -int 1

(in ML, the value of kPVPDFDefaultPageViewModeOption changes to 1 in Preview's plist, when selecting Single Page)