Can't help you with openpower.

But my experiences with 64-bit db2 & aix have been great.  Been running
mostly on older rs/6000s (H70s) and power4s (p630s).  In those
environments it has been extremely stable, extremely managable, and the
elimination of the two gbyte memory limit and extended memory
work-arounds has been a relief.

Plus, the combination of aix & power5 looks extremely promising.  I'm
planning to move several databases on separate servers to a single P570
running three lpars in 1-2 months.  The p570 is only a four-way, but
between its multi-core CPUs and virtualization (that will allow it to
constantly shift resources to the lpar that needs it the most) it'll
have the equivilent performance of three to four intel or power4
cpu-based four-way servers for my workload.

And given the increase in the rate of os patches these days, I'd prefer
to keep each database in its own os rather than on separate instances
within a single os.   Makes coordination and communication of os
reboots & upgrades much easier.  Lpars make this easy.

A good place to look for info is:
http://www-1.ibm.com/servers/eserver/pseries/

ken