Is there a way to get XCode to play nice with shared folders and allow multiple people to work on an iPhone app at the same time? Or are we going to need to switch to something more like SubEthaEdit or some other realtime-collaboration tool?
Xcode is know to play very badly with the file system. When managing a project, you have two option, to add the folders as group or as folder references.
If you add a group, then the folder will be mirrored into Xcode, but the group won't be in sync with the filesystem, that is you can move things within the group, delete the group, won't touch the FS, reverse will make Xcode lose the files.
The second option, to add a folder reference sounds very sweet, but is totally bugged. Folder references might not update properly, changes are not detected within the folders (if you add an Images folder reference to your resources and update the content, your resources won't be updated on next build, you have to clean&rebuild).
This is why you should either not use Xcode (we use vim + SCons here and are very happy with it) or you need to use an SCM (SCM can be coupled with other tools too, you SHOULD use an SCM anyway).
As for the SCM I recommend Mercurial, but there is also SVN and Git and a lot of others, I suggest you Google a bit and make your opinion.