[27]  linux-2.6.20-gentoo-r8 *
  [28]  linux-2.6.21-gentoo-r4
beast tim # eselect kernel set 27
beast tim # emerge -aP gentoo-sources

>>> These are the packages that would be unmerged:

 sys-kernel/gentoo-sources
    selected: 2.6.15 2.6.15-r2 2.6.15-r3 2.6.15-r4 2.6.15-r5 2.6.16-r3 2.6.16-r6 2.6.16-r7 2.6.16-r9 2.6.16-r11 2.6.16-r12 2.6.17-r4 2.6.17-r7 2.6.17-r8 2.6.18-r2 2.6.18-r3 2.6.18-r4 2.6.18-r5 2.6.18-r6 2.6.19-r5 2.6.16-r13 2.6.15-r1 2.6.20-r6 2.6.20-r7 2.6.20-r8 
   protected: 2.6.21-r4 
     omitted: none

beast tim # uname -a
Linux beast 2.6.20-gentoo-r8 #7 Fri Jun 22 00:25:30 BST 2007 i686 AMD Athlon(tm) XP 2500+ AuthenticAMD GNU/Linux