I've implemented new @live-rebuild and @module-rebuild package sets, and added them to the default sets.conf file for the next sys-apps/portage release (that will be 2.2_rc6). Here are the relevant sections from sets.conf:

# Installed ebuilds that inherit from known live eclasses.
[live-rebuild]
class = portage.sets.dbapi.InheritSet
world-candidate = False
inherits = cvs darcs git mercurial subversion

# Installed packages that own files inside /lib/modules.
[module-rebuild]
class = portage.sets.dbapi.OwnerSet
world-candidate = False
files = /lib/modules

Given the new InheritSet and OwnerSet classes, it's easy to define similar sets based on inherited eclasses and installed files, respectively.

Version 2.2_rc5 of sys-apps/portage adds support for EAPI="2_pre1" which has some changes in conditional USE dependency syntax. The previous experimental EAPI="2_pre0" value is no longer supported.

For more information about experimental EAPI features, refer to bug #233213 and also see the "Ebuild" chapter of the html documentation that is installed with portage when USE=doc is enabled. I've posted a copy of the EAPI="2_pre1" documentation here:

EAPI 2_pre1

  Helpers

    doman

      Language codes in file names are now used for path translation.

      Table 6.1. Man Page Path Translation

        Source        Destination
        foo.1         /usr/share/man/man1/foo.1
        foo.lang.1    /usr/share/man/lang/man1/foo.1

  USE Dependencies

    Unconditional USE Dependencies

      Table 6.2. Syntax Examples

        Example          Meaning
        foo[bar]         foo must have bar enabled
        foo[bar,baz]     foo must have both bar and baz enabled
        foo[-bar,baz]    foo must have bar disabled and baz enabled

    Conditional USE Dependencies

      Table 6.3. Syntax Examples

        Compact Form    Equivalent Expanded Form
        foo[bar?]       bar? ( foo[bar]  ) !bar? ( foo       )
        foo[!bar?]      bar? ( foo       ) !bar? ( foo[-bar] )
        foo[bar=]       bar? ( foo[bar]  ) !bar? ( foo[-bar] )
        foo[!bar=]      bar? ( foo[-bar] ) !bar? ( foo[bar]  )