Are developers allowed to use these resources freely in their own apps? I ask because "look and feel conflicts with first-party apps" has been grounds for rejection in the past.
mmm... Good point, nevertheless you can already use some resources (like the refresh button, or the favourites icon). But icon, popover background, textures, buttons, should be safe because they're not too branded.
For settings icon, it should be more prone to be rejected...
I don't think there really is a clearly defined boundary here.
I've worked on projects at past jobs where the designs demanded that we duplicate the look and feel of certain iOS system APIs and programs to a T. Apple passed those through approval without making a sound about those bits.
On the other hand, Tapbots has had an app rejected in the past for using a clock icon that looked a little like Apple's[1], back in 2009. And that wasn't an exact copy.
We're not going to get an official statement from Apple on this, so the best I can suggest is to tread with caution. Better to be inspired by than to flat out copy.