Solaris

You can import and upgrade the pool, the problem with this is that you won't be able to use it on Solaris 10 anymore if you upgrade the pool.

As Jlliagre indicated, it might not be the most stable of configurations if you do not upgrade the on disk format of the pool.

The on disk format for 2009.06 is older than the Solaris 10 U8 format, so you would need to update the OpenSolaris system using the dev repository to be able to use the pool if using U8 or quuivalent patches.

Yeah, I found some posting in a mailing list with a solution upgrading from production to devel, which is no option here.

Curios, why OpenSolaris' version is older than the one Solaris 10 uses...

Jiliagre, a simple "I don't know how to do that" had been a better answer.

Because, as I wrote, they are different branches with different update/release/integration schedules.
It won't. I was actually aware of the reason why it wasn't working but I would never have recommended the "fix" you have read elsewhere because you are running on a production environment and seem to lack backups.
What my reply was meaning is there is no commitment the on-disk format used by a given pool or zfs version number on OpenSolaris will be compatible with the very same pool or zfs version number when implemented later on Solaris 10.
The bottom line is I would never recommend hacking a supported system when that means turning it into an unsupported configuration.

Thou shalt be forgiven

Your one-single-sentence-answer was just a bit offending to someone who wants to rescue/access the data.

Got it. Sorry for being too direct. I also believe I was posting from my telephone which doesn't help elaborating answers ...