I upgraded to slackware64_current, and now I can't get any of my virtualbox machines to start.

I get "The VirtualBox kernel driver (vboxdrv) is either not loaded or there is a permission problem with /dev/veoxdrv. Please reinstall the kernal module by executing '/etc/init.d/vboxdrv.setup"

So I execute /etc/rc.d/rc.vboxdrv setup, and I get:

root@ook:/etc/rc.d# ./rc.vboxdrv setup
Stopping VirtualBox kernel modules ...done.
Uninstalling old VirtualBox DKMS kernel modules ...done.
Removing old VirtualBox netadp kernel module ...done.
Removing old VirtualBox netflt kernel module ...done.
Removing old VirtualBox kernel module ...done.
Trying to register the VirtualBox kernel modules using DKMS ...failed!
(Failed, trying without DKMS)
Recompiling VirtualBox kernel modules ...done.
Starting VirtualBox kernel modules ...failed!
(modprobe vboxdrv failed. Please use 'dmesg' to find out why)

dmesg tells me:

vboxdrv: version magic '2.6.38.7 SMP preempt mod_unload ' should be '2.6.38.7 SMP mod_unload '

I'm not quite sure what it is trying to tell me, and I have less of a clue what to do about it. I'm currently downloading 13.37, and I'm going roll my system back to that, as I have got to have my VMs running - I make a living on this box (yeah, yeah, I know, it was stupid to upgrade to slackware current on a box I make a living on...<sigh>....it's never failed me before, so I became complacent....)

I'm currently running slackware 64 current as of today (6-1-2011):

Linux ook 2.6.38.7 #3 SMP Sun May 22 16:21:50 CDT 2011 x86_64 AMD Phenom(tm) II X6 1100T Processor AuthenticAMD GNU/Linux

I have 8gb Ram, more than enough memory to go around.

Did you happen to upgrade from Slackware 32 bit? If so, you have to upgrade to the 64 bit version of VB.
Regards,
Bill

What happens when you issue modprobe vboxdrv by hand?

root@ook:/etc/rc.d# modprobe vboxdrv
FATAL: Error inserting vboxdrv (/lib/modules/2.6.38.7/misc/vboxdrv.ko): Invalid module format

No, I've always used the 64bit version. I even tried to install the 32 bit version but it detects the 64 bit OS and won't install.

Maybe I missed it, but did you re-install VirtualBox? Anytime you change the kernel you have to re-install VirtualBox and in the process it rebuilds the its kernel module.
Don't touch your ~.virtualbox directory, just re-install the file from Oracle and you should be fine.
I usually go to /opt/VirtualBox and run the uninstall.sh file, then go to where I store the VirtualBox-x.x.x-xxxxx-Linux_amd64.run package and run it.
You may also have to re-install the extension pack.

I did indeed reinstall virtual box. Alas, no joy.

I just finished a restore to 13.37, and virtualbox works just fine. My best guess is that something between 13.37 and current changed that breaks virtualbox. If I get the time, I may try upgrading to current again just to see if it does the same thing again. But for now - I work on this box and alas don't have time to play

I tried to install VB 3.0.x in Slackware-current, and no way.
Only install VB 4.0.x instead ...

What version of VB are you?

I used -Current (32 bit) and VB works fine here
If there's a change in the kernel, just run the VB installer again and it should work normally afterwards

cd to the /lib/modules/2.6.38.7/misc directory and run file on it, it should show as below:
I am running Virtual Box (installed using VirtualBox-4.0.8-71778-Linux_amd64.run) just fine in Slackware64-current.