Weeks ago, Nautilus 3.6 (3.5.9x) landed in Quantal Quetzal, introducing an overall refreshed application, featuring a refined look, monochrome icons, new features, as well as a solid development attention (aimed at and implementing a fresh Nautilus version).
Shortly, the 3.6 Nautilus was removed from Ubuntu 12.10 (at that moment, an in-progress Ubuntu version), due to the mentioned new changes and especially related to an improper integration with the Ubuntu desktop (including a menu housed by a "cog", HUD's inabillity to reach Nautilus' menus, etc).
Consequently, the developers removed the 3.6 version, reverting Nautilus to its 3.4.x version (similar to Ubuntu 12.04's), yet, they stated that the next Ubuntu development cycle (Raring Ringtail) is to probably receive an up-to-date (and/or desktop-integrated) Nautilus.
In an interview at UDS-R, Ubuntu Desktop Manger, Jason Warner, stated that, as related to the used GNOME version, the developers are to bear a "pragmatic" approach (in the Raring development cycle), "Usually we would go with the latest GNOME release, we are going to hold back to the stable GNOME release, that is also to include Nautilus".
Nautilus have gathered, across the latest Ubuntu cycles, a solid amount of interest, usually centered on if-when-why Nautilus is to be changed with a different file manager.
It seems that, for the 13.04 Ubuntu version, Nautilus is to remain the default file manager, "We want something rock solid,..., we will stick with Nautilus", decision paired with an upcoming work related to Nautilus' proper integration into the desktop.