I have this problem too - it's happening on roughly half my slice builds.

It seems to be due to the grub installation failing, here's what it shows for a build that won't boot:

 ** [out :: host-nwtn1.nwtn] 2013-03-17 10:52:33,044 DEBUG   : ['chroot', '/tmp/tmpgvQfyy', 'grub', '--device-map=/tmp/vmbuilder-grub/device.map', '--batch']
 ** [out :: host-nwtn1.nwtn] 2013-03-17 10:52:33,044 DEBUG   : stdin was set and it was a string: root (hd0,0)
 ** [out :: host-nwtn1.nwtn] setup (hd0)
 ** [out :: host-nwtn1.nwtn] EOT
 ** [out :: host-nwtn1.nwtn] 2013-03-17 10:52:33,197 DEBUG   :
 ** [out :: host-nwtn1.nwtn] 2013-03-17 10:52:33,197 DEBUG   :        [ Minimal BASH-like line editing is supported.   For
 ** [out :: host-nwtn1.nwtn] 2013-03-17 10:52:33,198 DEBUG   :          the   first   word,  TAB  lists  possible  command
 ** [out :: host-nwtn1.nwtn] 2013-03-17 10:52:33,198 DEBUG   :          completions.  Anywhere else TAB lists the possible
 ** [out :: host-nwtn1.nwtn] 2013-03-17 10:52:33,198 DEBUG   :          completions of a device/filename. ]
 ** [out :: host-nwtn1.nwtn] 2013-03-17 10:52:33,198 DEBUG   : grub> root (hd0,0)
 ** [out :: host-nwtn1.nwtn] 2013-03-17 10:52:33,198 DEBUG   : grub> setup (hd0)
 ** [out :: host-nwtn1.nwtn] 2013-03-17 10:52:33,198 DEBUG   :  Checking if "/boot/grub/stage1" exists... no
 ** [out :: host-nwtn1.nwtn] 2013-03-17 10:52:33,199 DEBUG   :  Checking if "/grub/stage1" exists... no
 ** [out :: host-nwtn1.nwtn] 2013-03-17 10:52:33,199 DEBUG   :
 ** [out :: host-nwtn1.nwtn] 2013-03-17 10:52:33,199 DEBUG   : Error 15: File not found
 ** [out :: host-nwtn1.nwtn] 2013-03-17 10:52:33,199 DEBUG   : grub> EOT

And here's what it shows for a build that does boot:

 ** [out :: host-nwtn1.nwtn] 2013-03-17 11:00:01,423 DEBUG   : ['chroot', '/tmp/tmpnUU59C', 'grub', '--device-map=/tmp/vmbuilder-grub/device.map', '--batch']
 ** [out :: host-nwtn1.nwtn] 2013-03-17 11:00:01,424 DEBUG   : stdin was set and it was a string: root (hd0,0)
 ** [out :: host-nwtn1.nwtn] setup (hd0)
 ** [out :: host-nwtn1.nwtn] EOT
 ** [out :: host-nwtn1.nwtn] 
 ** [out :: host-nwtn1.nwtn] 2013-03-17 11:00:01,599 DEBUG   :
 ** [out :: host-nwtn1.nwtn] 2013-03-17 11:00:01,599 DEBUG   :        [ Minimal BASH-like line editing is supported.   For
 ** [out :: host-nwtn1.nwtn] 2013-03-17 11:00:01,599 DEBUG   :          the   first   word,  TAB  lists  possible  command
 ** [out :: host-nwtn1.nwtn] 2013-03-17 11:00:01,599 DEBUG   :          completions.  Anywhere else TAB lists the possible
 ** [out :: host-nwtn1.nwtn] 2013-03-17 11:00:01,599 DEBUG   :          completions of a device/filename. ]
 ** [out :: host-nwtn1.nwtn] 2013-03-17 11:00:01,599 DEBUG   : grub> root (hd0,0)
 ** [out :: host-nwtn1.nwtn] 2013-03-17 11:00:01,600 DEBUG   : grub> setup (hd0)
 ** [out :: host-nwtn1.nwtn] 2013-03-17 11:00:01,600 DEBUG   :  Checking if "/boot/grub/stage1" exists... yes
 ** [out :: host-nwtn1.nwtn] 2013-03-17 11:00:01,600 DEBUG   :  Checking if "/boot/grub/stage2" exists... yes
 ** [out :: host-nwtn1.nwtn] 2013-03-17 11:00:01,600 DEBUG   :  Checking if "/boot/grub/e2fs_stage1_5" exists... yes
 ** [out :: host-nwtn1.nwtn] 2013-03-17 11:00:01,600 DEBUG   :  Running "embed /boot/grub/e2fs_stage1_5 (hd0)"...  21 sectors are embedded.
 ** [out :: host-nwtn1.nwtn] 2013-03-17 11:00:01,600 DEBUG   : succeeded
 ** [out :: host-nwtn1.nwtn] 2013-03-17 11:00:01,600 DEBUG   :  Running "install /boot/grub/stage1 (hd0) (hd0)1+21 p (hd0,0)/boot/grub/stage2 /boot/grub/menu.lst"... succeeded
 ** [out :: host-nwtn1.nwtn] 2013-03-17 11:00:01,600 DEBUG   : Done.
 ** [out :: host-nwtn1.nwtn] 2013-03-17 11:00:01,600 DEBUG   : grub> EOT


There are some other errors elsewhere in the process that may or may not be significant.  I've attached the full log for both the failed and successful builds.