I haven't had any experience with this explicitly, but if 2.6 is doing some-sort of "load-balancing" among IPs on the interface, there should be a way to turn this feature off. If not, you were on the right track with /etc/exports, but remember that you have to re-run exportfs (I prefer exportfs -av) on the filer for the changes to take effect. Then reboot the Solaris 2.6 client and see what happens.
Oh, I did the 'exportfs -av' many times. ;) No go.
I will try a more systematic approach when I can free up a machine. It was a mad frenzy today trying to get it to work, and I ended up giving up.
-Rasmus