Well, I finally found the correct set of Google search terms, so to answer my own question (DOH!), it seems to be possible to share code, at least between iPhone apps and possibly also iPhone and OS X apps within certain limits. You have to create a 'static library' and use 'cross-project references':
http://www.clintharris.net/2009/iphone-app-shared-libraries/
http://www.amateurinmotion.com/articles/2009/02/08/creating-a-static-library-for-iphone.html
http://zetetic.net/blog/2010/02/15/building-static-libraries-to-share-code-on-iphone-and-mac-os-x-projects/
http://weston-fl.com/blog/?p=808
Haven't tested any of this yet but it looks promising.
