Symbolic links are one of the great *nix things I love in OS X.  I wanted to share and sync my stencil library across a few of my computers.  Remove the folder /Library/Application\ Support/The\ Omni\ Group/OmniGraffle/Stencils. Then, one simple command does the trick. (Please note I keep my OmniGraffle Stencils folder inside of a folder call Personal in my dropbox).

ln -s ~/Dropbox/Personal/OmniGraffle/Stencils ~/Library/Application\ Support/The\ Omni\ Group/OmniGraffle/Stencils

Or, bring it down another level and sync Stencils, Scripts and Templates.

ln -s ~/Dropbox/Personal/OmniGraffle/ ~/Library/Application\ Support/The\ Omni\ Group/OmniGraffle