Failed to open a session for the virtual machine FreeBSD 8.2.
Failed to load VMMR0.r0 (VERR_SUPLIB_OWNER_NOT_ROOT).

Looking in the $LOCATION_OF_YOUR_VM/Logs/VBox.log file:

00:00:00.850 Oracle VM VirtualBox Extension Pack (Version: r0; VRDE Module: unusable because of 'The owner is not root: '/usr' (rc=VERR_SUPLIB_OWNER_NOT_ROOT)')
00:00:00.963 pdmR3LoadR0U: pszName="VMMR0.r0" rc=VERR_SUPLIB_OWNER_NOT_ROOT szErr="The owner is not root: '/usr'"

You get the error string "The owner is not root: '/usr'"

Looking at the permissions of /usr we find Houston has a problem.

ls -al /

dr-xr-xr-x.   2 root     root       12288 Aug 11 08:48 sbin
drwxr-xr-x.   7 root     root           0 Aug 11  2011 selinux
drwxr-xr-x.   2 root     root        4096 May 18 21:33 srv
drwxr-xr-x.  13 root     root           0 Aug 11  2011 sys
drwxrwxrwt.  64 root     root       12288 Aug 11 11:37 tmp
drwxrwxr-x.  13 postgres vboxusers   4096 Aug  3 16:35 usr
drwxr-xr-x.  19 root     root        4096 Jul 11 20:47 var

# attempted fix but I don't know if this is creating other problems
chown root:root /usr -R

# still didn't work so had to remove and re-install
# make sure you stop all instances of virtualbox first.
yum remove VirtualBox-4.1-4.1.0_73009_fedora15-1.x86_64
yum install VirtualBox-4.1-4.1.0_73009_fedora15-1.x86_64


# had to go for the -R option because the problem continued
# down the tree as you can see here.
ls -al /usr/ 
total 288
drwxrwxr-x.  13 root     root        4096 Aug  3 16:35 .
dr-xr-xr-x.  27 root     root        4096 Aug 11 07:27 ..
drwxrwxr-x.   2 postgres vboxusers  73728 Aug 11 08:48 bin
drwxr-xr-x.   2 root     root        4096 May 18 21:33 etc
drwxr-xr-x.   2 root     root        4096 May 18 21:33 games
drwxr-xr-x.  35 root     root        4096 Aug 10 19:35 include
drwxrwxr-x.  50 postgres vboxusers  20480 Aug 11 08:48 lib
dr-xr-xr-x. 175 root     root      126976 Aug 11 08:48 lib64
drwxr-xr-x.  31 root     root       12288 Aug 11 08:48 libexec
drwxr-xr-x.  12 root     root        4096 May 18 21:33 local
dr-xr-xr-x.   2 root     root       12288 Aug 11 08:48 sbin
drwxrwxr-x. 326 postgres vboxusers  12288 Aug  3 16:35 share
drwxr-xr-x.   4 root     root        4096 Aug  2 08:41 src
lrwxrwxrwx.   1 root     root          10 Jul 11 17:43 tmp -> ../var/tmp

I have an (unproven) suspicion that a recent test install of the xTuple Postbooks (which installs it's own copy of Postgres and add's the postgres user) may be responsible for the Perm change.