This is a fairly frequently asked question inside Sun, and now that the biweekly builds are available outside Sun, there are probably more people who are interested.  If you want to create a Boot Environment with a specific build (not the latest one) in OpenSolaris, here are the steps to use:

beadm create snv122
beadm mount snv122 /mnt
pkg -R /mnt install pkg:/entire@0.5.11-0.122
bootadm update-archive -R /mnt
beadm umount snv122
beadm activate snv122

Caveat Emptor and all that stuff.  This recipe comes with no warranty, but I’d be glad to make it more accurate or up-to-date if you let me know of improvements.