CURRENT ( > r249381): can't find boot partition on GPT disk

Trying to boot a kernel > r249381 fails and I see on the console the
loader prompt at "mountroot". Obviously, the bootloader doesn't find the
root/boot partition. On all FreeBSD 10 boxes I have this phenomenon is
the same, all boxes have GPT (UFS) partitions to boot from and set GPT
labels to address the partitions:

/etc/fstab looks like

 # Device        Mountpoint      FStype  Options Dump    Pass#
/dev/gpt/root   /               ufs     rw      1       1
/dev/gpt/swap   none            swap    sw      0       0
tmpfs           /tmp            tmpfs   rw,size=4294967296      0
0
/dev/gpt/var    /var            ufs     rw      2       2
tmpfs           /var/run        tmpfs   rw,size=4294967296      0
0
/dev/gpt/var.tmp        /var/tmp                ufs     rw      2
2
/dev/gpt/compat         /compat         ufs     rw      2       2
/dev/gpt/usr            /usr            ufs     rw      2       2
/dev/gpt/usr.src        /usr/src                ufs     rw      2
2
/dev/gpt/usr.obj        /usr/obj                ufs     rw      2
2
/dev/gpt/usr.local      /usr/local              ufs     rw      2
2


Booting the old kernel works fine.

regards,

Oliver