You probably know that you can add your own code snippets in Xcode 4.
Those snippets are stored in
~/Library/Developer/Xcode/UserData/CodeSnippets. You can create a symlink in your Dropbox folder to keep your snippets backed up and synced with your other computers:
ln -s ~/Library/Developer/Xcode/UserData/CodeSnippets ~/Dropbox/XcodeSnippets
~/Dropbox/ with your actual Dropbox folder location)