Will Debian (GNU/kFreeBSD) + official ZFS support be a stable configuration?

I'm not sure it would be a big win over OpenSolaris. The current ZFS implementation in FreeBSD stable (8.1) trails the OpenSolaris version by quite a big chunk. The code in the development tree (9.0)is a lot more current, but I don't know how stable that is. Because of the version difference, you lose out on features like dedupe, which can be huge, depending on your usage, and you may have issues downgrading your zpools.

you have to ask yourself how many people are running the debian/bsd hybrid...and how many of them rely on it to be stable

this project has always struck me as a fun idea and experiment, but i wouldn't presume to rely on it

No, that is a toy to play around with - a nice one though.

Note that FreeBSD has no OFED stack that could get your IB partition running ...

Debian GNU/kFreeBSD is not a toy anymore! With Squeeze is will be a stable Debian port, with full security support and updates.

Sure, the release is labeled as "technology preview" but this is because on the desktop it's not fully usable yet (there are many bugs that populate GNOME/KDE libraries and other important desktop components).

OTOH on the server it's perfectly fine. We're using it on production already (as backup server, not a big deployment mind you) with no serious trouble.