I’ve come across quite a few pdf/document managers for OSX. iPapers is a bit different as it is centered around PubMed PDF documents. You can search from within the app or easily import documents if they are named using the PMID. iPapers retrieves the metadata from PubMed for you. It allows you to filter papers by author, title, date, etc. A rather narrow use application but an idea that might be worth expanding.
I still think citation management will likely move to the web but organizers like this may have it’s place, especially if you can search across large indexes in a single, simple interface. I’ve seen posts on various blogs/irc/email talking about the idea of creating an open-source refworks clone that can be tightly integrated with metasearch offerings, etc. Will be interesting to see if anything comes of it.
I think the importance of web services, coins/openurl, uri’s etc will come to the forefront when these sort of applications begin to be built. It will likely be of some complexity in order to get it to be simple for the users. I’ll leave the debate over whether it should be simple to ngc4lib.