6-cur doesn't find ad0p3 with MBR and GPT on the same disk

Hello,

I upgraded a experimenting box from 5.4 to 6 with the result that 6 can't find 
my GPT slices.
The disk additionally contains an MBR for booting, I'm on x86...

Is there any trick to convince 6 to accept GPT slices after mounting root from 
the same disk, traditonally from ad0s1a, so I can continue mounting /usr 
a.s.o. which are on ad0p3?

With the old 5.4 kernel it was no problem, but userland is already -current so 
I cannot login anymore :(

Thanks,

-Harry