Already on my check list
I switched all my machines away from M$ 5 months ago to Ubuntu, only my server still running SBS2003. And after a quick check of Open Solaris I am very positive that I will switch, at least my server to it, in the near future.
The lack of expandable ZFS arrays is the only problem I am considering as important, but its not an impossible problem to overcome, everything else worked without a problem.
However I am a small shop, bigger installations have more things to think about.
Congrats to Sun for a great effort.